Migrate to an even more async design

Migrate to an even more async design

This monster commit replaces our database engine by SQLAlchemy so that
we can utilise eager loading and don't have to implement this on our
own. Although Jinja is a lot more flexible and allows us to
asynchronically fetch any data, we cannot really have async properties.
Therefore it is a good idea to have some eager loading happening when we
fetch certain objects that will always depend on some other objects.

This has been a major rewrite of the backend which still isn't done, but
I needed to commit this somehow. There was little point it doing it
gradually so here is this mountain of a commit. Enjoy.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>