Resolve "Convert list of forum threads to Vue"

What does this MR do?

Convert list of forum threads to Vue, and implement better deeplink handling for posts.

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

pretty confident. The mobile view is something we'll have to look at sometime.

Links to related issues

• !1609 (merged)
• closes #962 (closed)
• closes #937 (closed)
• closes #764 (closed)
• closes #86 (closed)
• &22 (closed)
• might fix #930 (closed) but I'm not sure since I can't test it

How to test

• write some new forum threads, or use the existing ones (use different accounts to test avatar handling)

• write replies to some, but not all of them (use different accounts to test avatar handling)

• check the list of forum threads in working groups and regions

• also check ambassador forums for regions

• verify that all links work as expected etc

• to test the pagination, you can reduce default="20" for limit in ForumRestController:listThreadsAction to a lower number

• inside a thread, click on the date for some posts, the URL should change and scrolling should happen

• reload the page / open it in a new tab etc, see if it behaves as expected

• if you are able to reproduce #930 (closed) (I am not) please check if this also fixed now

Screenshots (if applicable)

The only (intentional) visual change is a slight style modification to the forum search.

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
• add a short text that can be used in the release notes forDev
• 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.