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.

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.

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

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

This PR adds the missing user_mapping_provider section in oidc.md

Signed-off-by: Christopher Rücker <chris-ruecker@protonmail.com>

Signed-off-by: Emelie <em@nao.sh>

During login, if there are multiple IdPs enabled, offer the user a choice of
IdPs.

Removes the trailing slash with causes issues with matrix.to/Element.

Adds the redacts endpoint to workers that have the client listener.

This makes the CAS handler look more like the SAML/OIDC handlers:

* Render errors to users instead of throwing JSON errors.
* Internal reorganization.

Adds a new setting `email.invite_client_location` which, if defined, is
passed to the identity server during invites.

This adds an admin API that allows a server admin to get power in a room if a local user has power in a room. Will also invite the user if they're not in the room and its a private room. Can specify another user (rather than the admin user) to be granted power.

Co-authored-by: Matthew Hodgson <matthew@matrix.org>

The final part (for now) of my work to implement a username picker in synapse itself. The idea is that we allow
`UsernameMappingProvider`s to return `localpart=None`, in which case, rather than redirecting the browser
back to the client, we redirect to a username-picker resource, which allows the user to enter a username.
We *then* complete the SSO flow (including doing the client permission checks).

The static resources for the username picker itself (in 
https://github.com/matrix-org/synapse/tree/rav/username_picker/synapse/res/username_picker)
are essentially lifted wholesale from
https://github.com/matrix-org/matrix-synapse-saml-mozilla/tree/master/matrix_synapse_saml_mozilla/res. 
As the comment says, we might want to think about making them customisable, but that can be a follow-up. 

Fixes #8876.

Move it from the federation section to the server section to match
ip_range_blacklist.

Spam checker modules can now provide async methods. This is implemented
in a backwards-compatible manner.

Deprecate both APIs in favour of the Delete Room API.

Related: #8663 and #8810

This defaults `ip_range_blacklist` to reserved IP ranges and also adds an
`ip_range_whitelist` setting to override it.

Related: #8810
Also a few small improvements.

Signed-off-by: Dirk Klimpel <dirk@klimpel.org>

Authentication is done by checking a shared secret provided
in the Synapse configuration file.

This was broken in #8801.

the constructor is called with a `module_api`.

Replaces the `federation_ip_range_blacklist` configuration setting with an
`ip_range_blacklist` setting with wider scope. It now applies to:

* Federation
* Identity servers
* Push notifications
* Checking key validitity for third-party invite events

The old `federation_ip_range_blacklist` setting is still honored if present, but
with reduced scope (it only applies to federation and identity servers).

A word got removed accidentally in 83434df3.

Add a config option to change whether unread push notification counts are per-message or per-room (#8820)

This PR adds a new config option to the `push` section of the homeserver config, `group_unread_count_by_room`. By default Synapse will group push notifications by room (so if you have 1000 unread messages, if they lie in 55 rooms, you'll see an unread count on your phone of 55).

However, it is also useful to be able to send out the true count of unread messages if desired. If `group_unread_count_by_room` is set to `false`, then with the above example, one would see an unread count of 1000 (email anyone?).

These are now only available via `/_synapse/admin/v1`.

This PR updates the push config's formatting to better align with our [code style guidelines](https://github.com/matrix-org/synapse/blob/develop/docs/code_style.md#configuration-file-format).

Abstracts the SAML and OpenID Connect code which attempts to regenerate
the localpart of a matrix ID if it is already in use.

Clarify that the list media API only shows media from unencrypted events.

Some hopefully-useful notes on setting up a turnserver.