Ensure support users can be registered even if MAU limit is reached

This allows support users to be created even on MAU limits via
the admin API. Support users are excluded from MAU after creation,
so it makes sense to exclude them in creation - except if the
whole host is in disabled state.

Signed-off-by: Jason Robinson <jasonr@matrix.org>

This is a redo of https://github.com/matrix-org/synapse/pull/5897 but with `id_access_token` accepted.

Implements [MSC2134](https://github.com/matrix-org/matrix-doc/pull/2134) plus Identity Service v2 authentication ala [MSC2140](https://github.com/matrix-org/matrix-doc/pull/2140).

Identity lookup-related functions were also moved from `RoomMemberHandler` to `IdentityHandler`.

Allow use of different ratelimits for admin redactions.

* remove some unused code
* make things which were constants into constants for efficiency and clarity

Some small fixes to `room_member.py` found while doing other PRs.

1. Add requester to the base `_remote_reject_invite` method.
2. `send_membership_event`'s docstring was out of date and took in a `remote_room_hosts` arg that was not used and no calling function provided. 

Co-Authored-By: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

Two things I missed while implementing [MSC2140](https://github.com/matrix-org/matrix-doc/pull/2140/files#diff-c03a26de5ac40fb532de19cb7fc2aaf7R80).

1. Access tokens should be provided to the identity server as `access_token`, not `id_access_token`, even though the homeserver may accept the tokens as `id_access_token`.
2. Access tokens must be sent to the identity server in a query parameter, the JSON body is not allowed.

We now send the access token as part of an `Authorization: ...` header, which fixes both things.

The breaking code was added in https://github.com/matrix-org/synapse/pull/5892

Sytest PR: https://github.com/matrix-org/sytest/pull/697

This is useful to allow room admins to quickly deal with a large number
of abusive messages.

Remove origin parameter from add_display_name_to_third_party_invite and add params to docstring (#6010)

Another small fixup noticed during work on a larger PR. The `origin` field of `add_display_name_to_third_party_invite` is not used and likely was just carried over from the `on_PUT` method of `FederationThirdPartyInviteExchangeServlet` which, like all other servlets, provides an `origin` argument.

Since it's not used anywhere in the handler function though, we should remove it from the function arguments.

Use account_threepid_delegate for 3pid validation

Only count real users when checking for auto-creation of auto-join room

Censor redactions in DB after a month

Signed-off-by: Jason Robinson <jasonr@matrix.org>

Add opentracing span for HTTP push

Signed-off-by: Jason Robinson <jasonr@matrix.org>

Signed-off-by: Jason Robinson <jasonr@matrix.org>

Signed-off-by: Jason Robinson <jasonr@matrix.org>

Previously if the first registered user was a "support" or "bot" user,
when the first real user registers, the auto-join rooms were not
created.

Fix to exclude non-real (ie users with a special user type) users
when counting how many users there are to determine whether we should
auto-create a room.

Signed-off-by: Jason Robinson <jasonr@matrix.org>

`sid` is required to be part of `three_pid_creds`. We were 500'ing if it wasn't provided instead of returning `M_MISSING_PARAM`.

Correctly handle non-bool m.federate flag