* Consistently use room_id from federation request body

Some federation APIs have a redundant `room_id` path param (see
https://github.com/matrix-org/matrix-doc/issues/2330). We should make sure we
consistently use either the path param or the body param, and the body param is
easier.

* Kill off some references to "context"

Once upon a time, "rooms" were known as "contexts". I think this kills of the
last references to "contexts".

This was broken in #8801.

This was broken in #8801 when abstracting code shared with OIDC.

After this change both SAML and OIDC have a concept of
grandfathering users, but with different implementations.

The idea here is to abstract out all the conditional code which tests which
methods a given password provider has, to provide a consistent interface.

This PR adds a `room_version` argument to the `RestHelper`'s `create_room_as` function for tests. I plan to use this for testing knocking, which currently uses an unstable room version.

The spec requires synapse to support `identifier` dicts for `m.login.password`
user-interactive auth, which it did not (instead, it required an undocumented
`user` parameter.)

To fix this properly, we need to pull the code that interprets `identifier`
into `AuthHandler.validate_login` so that it can be called from the UIA code.

Fixes #5665.

Fix a minor bug where we would offer "m.login.password" login if a custom auth provider supported it, even if password login was disabled.

These things seemed to be completely untested, so I added a load of tests for
them.

This test was broken by #8565. It doesn't need to set set `self.clock`
here anyway - that is done by `setUp`.

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?).

Changes `@cache_in_self` to use underscore-prefixed attributes.

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

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

Checks that the localpart returned by mapping providers for SAML and
OIDC are valid before registering new users.

Extends the OIDC tests for existing users and invalid data.

* Consistently use room_id from federation request body

Some federation APIs have a redundant `room_id` path param (see
https://github.com/matrix-org/matrix-doc/issues/2330). We should make sure we
consistently use either the path param or the body param, and the body param is
easier.

* Kill off some references to "context"

Once upon a time, "rooms" were known as "contexts". I think this kills of the
last references to "contexts".

This was broken due to #8617 and #8761.

De-duplicates code between the SAML and OIDC implementations.

remove the stubbing out of `request.process`, so that `requestReceived` also renders the request via the appropriate resource.

Replace render() with a stub for now.

the preview resource is mointed at preview_url, not url_preview

FakeChannel has everything we need, and this more accurately models the real
flow.

The root resource isn't necessarily a JsonResource, so rename this method
accordingly, and update a couple of test classes to use the method rather than
directly manipulating self.resource.

Some tests want to set some custom HTTP request headers, so provide a way to do
that before calling requestReceived().

Where we want to render a request against a specific Resource, call the global
make_request() function rather than the one in HomeserverTestCase, allowing us
to pass in an appropriate `Site`.

Add `GET /_synapse/admin/v1/statistics/users/media` to get statisics about local media usage by users.
Related to #6094
It is the first API for statistics.
Goal is to avoid/reduce usage of sql queries like [Wiki analyzing Synapse](https://github.com/matrix-org/synapse/wiki/SQL-for-analyzing-Synapse-PostgreSQL-database-stats

)

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

Add `displayname` to Shared-Secret Registration for admins to `POST /_synapse/admin/v1/register`

I idly noticed that these lists were out of sync with each other, causing us to miss a table in a test case (`local_invites`). Let's consolidate this list instead to prevent this from happening in the future.

Fixes #4042

We don't always need the full power of a DeferredCache.

Fixes #6755

* Fix unit tests

* Newsfile