* group together key validity refactors

Improve documentation of monthly active user blocking and mau_trial_days

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

It takes at least 20 minutes to work through the long_retries schedule (11
attempts, each with a 60 second timeout, and 60 seconds between each request),
so if the notary server isn't returning within the timeout, we'll just end up
blocking whatever request is happening for 20 minutes.

Ain't nobody got time for that.

Fixes for the key-notary server

... else we're guaranteed to time out.

When handling incoming federation requests, make sure that we have an
up-to-date copy of the signing key.

We do not yet enforce the validity period for event signatures.

This reverts commit 4bd67db1.

Stop overwriting server keys with other keys

Limit displaynames and avatar URLs

These end up in join events everywhere, so let's limit them.

Fixes #5079

Rewrite get_server_verify_keys, again.

Fix a bug where we would discard a key result which the origin server is no
longer returning. Fixes #5305.

Allow configuring a range for the account validity startup job

Remove some pointless exception handling

use attr.s for VerifyKeyRequest