Building mx-puppet-discord appears to have been broken for a while.

Related to:
- https://github.com/matrix-discord/mx-puppet-discord/issues/201
- https://github.com/matrix-discord/mx-puppet-discord/issues/202
- https://github.com/matrix-discord/mx-puppet-discord/issues/203

We'd rather use a fork that is maintained better and by someone who
cares about whether their software works or not, so we'll be using the
Beeper-maintained for from now on.

In the future, we should probably do the same for the Slack bridge
which is also part of the same monorepo
(https://gitlab.com/beeper/mx-puppet-monorepo).

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1070

Our old (base-path -> data-path) SQLite migration can't work otherwise.

It's probably not necessary to keep it anymore, but since we still do,
at least we should take care to ensure it works.

Should have been part of 149872e00c

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/716

This patch makes us use more fully-qualified container image names
(either prefixed with docker.io/ or with localhost/).

The latter happens when self-building is enabled.

We've recently had issues where if an image was removed manually
and the service was restarted (making `docker run` fetch it from Docker Hub, etc.),
we'd end up with a pulled image, even though we're aiming for a self-built one.
Re-running the playbook would then not do a rebuild, because:
- the image with that name already exists (even though it's something
else)
- we sometimes had conditional logic where we'd build only if the git
repo changed

By explicitly changing the name of the images (prefixing with localhost/),
we avoid such confusion and the possibility that we'd automatically pul something
which is not what we expect.

Also, I've removed that condition where building would happen on git
changes only. We now always build (unless an image with that name
already exists). We just force-build when the git repo changes.