When persisting events we calculate new stream orderings up front.
Before we notify about an event all events with lower stream orderings
must have finished being persisted.

This PR moves the assignment of stream ordering till *after* calculated
the new current state and split the batch of events into separate chunks
for persistence. This means that if it takes a long time to calculate
new current state then it will not block events in other rooms being
notified about.

This should help reduce some global pauses in the events stream which
can last for tens of seconds (if not longer), caused by some
particularly expensive state resolutions.

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.

The double posting is really annoying, and I don't think anyone is
actually reading them. The commit statuses should give a good summary
and will link to a full report.

Don't recreate current_state_events.membership column