Allow overriding the server_notices user's avatar

probably should have done this in the first place, like @turt2live suggested.

Use the localpart in the consent uri

Docs on consent bits

Support for putting %(consent_uri)s in messages

... because it's shorter.

Make it possible to put the URI in the error message and the server notice that
get sent by the server

Block attempts to send server notices to remote users

Don't support limitless pagination

The pagination storage function supported not specifiying a limit on the
number of events returned. This was triggered when using the search or
context API with a limit of zero, which the storage function took to
mean not being limited.

Add a 'has_consented' template var to consent forms

Fix dependency on jinja2

fixes #3260

... to reduce the risk of somebody introducing an html injection attack...

Delay the import of ConsentResource, so that we can get away without jinja2 if
people don't have the consent resource enabled.

Fixes #3259

Reject attempts to send event before privacy consent is given

Returns an M_CONSENT_NOT_GIVEN error (cf
https://github.com/matrix-org/matrix-doc/issues/1252) if consent is not yet
given.

Send users a server notice about consent

... and have the sync endpoints call it directly rather than obsure indirection
via PresenceHandler