Handle TimelineBatch being limited and empty.

Part of fixing matrix-org/sytest#652

Sytest PR: matrix-org/sytest#667

This hopefully addresses #5407 by gracefully handling an empty but
limited TimelineBatch. We also add some logging to figure out how this
is happening.

Account validity: allow defining HTML templates to serve the user on account renewal attempt

Return 502 not 500 when failing to reach any remote server.

Fixes #5803

Handle incorrectly encoded query params correctly

Validate well known state events are state events.

Don't allow clients to send tombstones that reference the same room

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

Deny redaction of events in a different room.

Handle RequestSendFailed exception correctly in more places.

Explicitly check that tombstone is a state event before notifying.

Lets disallow sending things like memberships, topics etc as non-state
events.

We already correctly filter out such redactions, but we should also deny
them over the CS API.

This is intended as an amendment to #5674 as using M_UNKNOWN as the errcode makes it hard for clients to differentiate between an invalid password and a deactivated user (the problem we were trying to solve in the first place).

M_UNKNOWN was originally chosen as it was presumed than an MSC would have to be carried out to add a new code, but as Synapse often is the testing bed for new MSC implementations, it makes sense to try it out first in the wild and then add it into the spec if it is successful. Thus this PR return a new M_USER_DEACTIVATED code when a deactivated user attempts to login.

Share SSL options for well-known requests

Disable codecov reports to GH comments.