unify thread/theme/topic to thread

Closes #840 (closed)

What does this MR do?

Theme, thread and topic seem to be used often for the same thing: a thread in a forum.

Rename this to thread where possible (not in the database)

How confident are you it won't break things if deployed?

the linter doesn't complain!

How to test

In theory, any of the changed codepaths:

• Are forum members still notified about new posts properly per mail?
• Are admins of a forum still notified properly per mail when they have to accept a new thread?
• Can threads be still unfollowed in the settings?
• Do dashboard updates work as well as before?

etc, but in practice reviewing the changes closely should be enough.

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md