Make /sync return heroes if room name or canonical alias are empty

Validate federation server TLS certificates by default.

add a script to generate new signing_key files

Include left members in room summaries' heroes

Implement room v5 which enforces signing key validity

Associate a request_name with each verify request, for logging

Fix `federation_custom_ca_list` configuration option.

Previously, setting this option would cause an exception at startup.

* Update _matrix/client/versions to reference support for r0.5.0

* Fix background updates to handle redactions/rejections

In background updates based on current state delta stream we need to
handle that we may not have all the events (or at least that
`get_events` may raise an exception).

fixes #4951

Fix handling of failures when calling /event_auth.

* group together key validity refactors

Improve documentation of monthly active user blocking and mau_trial_days

Do user_id != me checks before deciding whether we should pick heroes from the joined members or the parted ones

Implements [MSC2077](https://github.com/matrix-org/matrix-doc/pull/2077) and
fixes #5247 and #4364.

Also:
* rename VerifyKeyRequest->VerifyJsonRequest
* calculate key_ids on VerifyJsonRequest construction
* refactor things to pass around VerifyJsonRequests instead of 4-tuples

Remove some spurious stuff, clarify some other stuff

it's a bit confusing

Fixes a regression introduced in #5335.

FederationClient.get_pdu is called in a loop to fetch a batch of PDUs. A
failure to fetch one should not result in a failure of the whole batch. Add the
missing `continue`.

We have too many things called get_event, and it's hard to figure out what we
mean. Also remove some unused params from the signature, and add some logging.

Make account validity renewal emails work when email notifs are disabled

Ignore room state with null bytes in for room stats