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

turns out we need to reuse this, so it's better in the config class.

When a user first syncs, we will send them a server notice asking them to
consent to the privacy policy if they have not already done so.

we're going to use it for the version we require too.

Fix logcontext resource usage tracking

Add in flight request metrics

Stop the transaction cache caching failures

Tighter filtering for user_daily_visits

ONE DAY I WILL PURGE THE WORLD OF THIS EVIL

The transaction cache has some code which tries to stop it caching failures,
but if the callback function failed straight away, then things would happen
backwards and we'd end up with the failure stuck in the cache.

Add lxml to docker image for web previews

This tracks CPU and DB usage while requests are in flight, rather than
when we write the response.

fix psql compatability bug

Signed-off-by: Paul Tötterman <paul.totterman@iki.fi>