Explicitly add something to session
Explicitly add something to session

This is necessary because `cascade_backrefs=False` disables the
implicit mechanism of attaching objects indirectly to our `session`.

Doing things like these explicitly is also beneficial because we can
now judge what goes into the session without having to be familiar
with implementation detais of all relevant `relationship`s

Refs #471

agdsn/pycroft,agdsn/pycroft,agdsn/pycroft,agdsn/pycroft,agdsn/pycroft