Add DSL and deterministic test data for forums
For now, all other members of the forum appear as strangers and the forum is shared with nobody, since none of them is connected with a real contact. For this to work, we would probably need to revise the whole deterministic test data creation, so that we can re-use contact from the private message generation inside forums as well. Probably something for a follow-up issue.
Closes #407 (closed)