Make titles of forum posts searchable

Closes #99 (closed)

What does this MR do?

Adds

• an endpoint GET /search/forum/{groupId} that searches for a query in forum thread titles
• a search field on forum pages

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

Should work

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Login as foodsaver
3. Go to a forum in a region or work group
4. Search for a forum title
5. Click on one of the results, which should redirect to the thread

This should work for normal region forums, ambassador forums, and work group forums.

Screenshots (if applicable)

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.