...instead of just creating the exception object and doing nothing with it.

* Factor out a common TestHtmlParser

Looks like I'm doing this in a few different places.

* Improve OIDC login test

Complete the OIDC login flow, rather than giving up halfway through.

* Ensure that OIDC login works with multiple OIDC providers

* Fix bugs in handling clientRedirectUrl

 - don't drop duplicate query-params, or params with no value
 - allow utf-8 in query-params

setuptools 51.0.0 dropped support for Python 3.5.

We do this by allowing a single iteration to process multiple rooms at a
time, as there are often a lot of really tiny rooms, which can massively
slow things down.

I don't think there's any need to use canonicaljson here.

Fixes: #4475.

This is the final step for supporting multiple OIDC providers concurrently.

First of all, we reorganise the config so that you can specify a list of OIDC providers, instead of a single one. Before:

    oidc_config:
       enabled: true
       issuer: "https://oidc_provider"
       # etc

After:

    oidc_providers:
     - idp_id: prov1
       issuer: "https://oidc_provider"

     - idp_id: prov2
       issuer: "https://another_oidc_provider"

The old format is still grandfathered in.

With that done, it's then simply a matter of having OidcHandler instantiate a new OidcProvider for each configured provider.

Protecting media stops it from being quarantined when
e.g. all media in a room is quarantined. This is useful
for sticker packs and other media that is uploaded by
server administrators, but used by many people.

Previously this code generated unreferenced `Deferred` instances
which caused "Unhandled Deferreds" errors to appear in error
situations.

`distutils` is pretty much deprecated these days, and replaced with
`setuptools`. It's also annoying because it's you can't `pip install` it, and
it's hard to figure out which debian package we should depend on to make sure
it's there.

Since we only use it for a tiny function anyway, let's just vendor said
function into our codebase.

* make the OIDC bits of the test work at a higher level - via the REST api instead of poking the OIDCHandler directly.
* Move it to test_login.py, where I think it fits better.

Again in preparation for handling more than one OIDC provider, add a new caveat to the macaroon used as an OIDC session cookie, which remembers which OIDC provider we are talking to. In future, when we get a callback, we'll need it to make sure we talk to the right IdP.

As part of this, I'm adding an idp_id and idp_name field to the OIDC configuration object. They aren't yet documented, and we'll just use the old values by default.

We passed in a graph to `sorted_topologically` which didn't have an
entry for each node (as we dropped nodes with no edges).

t was doing a sequential scan on `destination_rooms`, which took
minutes.

The idea here is that we will have an instance of OidcProvider for each
configured IdP, with OidcHandler just doing the marshalling of them.

For now it's still hardcoded with a single provider.

A reactor was being passed instead of a whitelist for the BlacklistingAgentWrapper
used by the WellyKnownResolver. This coulld cause exceptions when attempting to
connect to IP addresses that are blacklisted, but in reality this did not have any
observable affect since this code is not used for IP literals.

If a user tries to do UI Auth via SSO, but uses the wrong account on the SSO
IdP, try to give them a better error.

Previously, the UIA would claim to be successful, but then the operation in
question would simply fail with "auth fail". Instead, serve up an error page
which explains the failure.

* Add complete test for UI-Auth-via-SSO.

* review comments

Removes a bare `except Exception` clause and replaces it with
catching a specific exception around the portion that might throw.

This checks that the domain given to `DomainSpecificString.is_valid` (e.g.
`UserID`, `RoomAlias`, etc.) is of a valid form. Previously some validation
was done on the localpart (e.g. the sigil), but not the domain portion.

Some light refactoring of OidcHandler, in preparation for bigger things:

  * remove inheritance from deprecated BaseHandler
  * add an object to hold the things that go into a session cookie
  * factor out a separate class for manipulating said cookies

This only applies if the user's data is to be erased.

If we have integrations with multiple identity providers, when the user does a UI Auth, we need to redirect them to the right one.

There are a few steps to this. First of all we actually need to store the userid of the user we are trying to validate in the UIA session, since the /auth/sso/fallback/web request is unauthenticated.

Then, once we get the /auth/sso/fallback/web request, we can fish the user id out of the session, and use it to look up the external id mappings, and hence pick an SSO provider for them.

It is expected that the proxy would be on a private IP address so the
configured proxy should be connected to regardless of the IP range
blacklist.

Homeserver.get_ip_from_request() used to be a bit more complicated, but now it is totally redundant. Let's get rid of it.

SynapseRequest is in danger of becoming a bit of a dumping-ground for "useful stuff relating to Requests",
which isn't really its intention (its purpose is to override render, finished and connectionLost to set up the 
LoggingContext and write the right entries to the request log).

Putting utility functions inside SynapseRequest means that lots of our code ends up requiring a
SynapseRequest when there is nothing synapse-specific about the Request at all, and any old
twisted.web.iweb.IRequest will do. This increases code coupling and makes testing more difficult.

In short: move get_user_agent out to a utility function.

For remote users, only the rooms which the server knows about are returned.
Local users have all of their joined rooms returned.