* Add helpers for getting prev and auth events

This is in preparation for allowing the event format to change between
room versions.

Clean up event accesses and tests

This is in preparation to refactor FrozenEvent to support different
event formats for different room versions

Fix locked upsert on device_lists_remote_cache

Fix logic error that prevented guests from seeing the privacy policy

Fix None exception in state res v2

The indexes on device_lists_remote_extremeties can be unique, and they
therefore should, to ensure that the db remains consistent.

We can rely on the application-level per-user linearizer.

Fix build of Docker image with docker-compose

Add m.login.terms to the registration flow

Add STATE_V2_TEST room version

Attempt to move room aliases on room upgrades

Fix typo in docker-compose.yml

I found these helpful in debugging my room upgrade tests.

Attempt to fix tox installs

Signed-off-by: Jonas Schürmann <jonasschuermann@aol.de>

It seems that, at some point, the ability to run tox on old servers (with old
setuptools) got broken - and it was only working on our Jenkins instance by
dint of reusing the tox environments.

Let's try to get tox to do the right thing, and remove the guff from
jenkins/prepare_synapse.sh.

(There is a separate question about whether the jenkins builds should be using
tox to prepare the virtualenv at all here, but that is somewhat orthogonal).