Remove forum topic subscriptions when leaving a group

Closes #593 (closed)

What does this MR do?

Removes subscriptions to forum topics if

• a user leaves a working group
• the member list of a working group or region was edited and users are kicked out

by removing entries from fs_theme_follower.

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

Works locally, but I'm not absolutely sure that this covers all cases and nothing is deleted unintentionally

Links to related issues

possibly also #23 (closed)

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))