Modernize forum threadlist layout

What does this MR do?

• modernize layout (flexbox instead of float/clear-based) of forum threadlist
• drastically improve responsive behavior on medium and small screen widths
• add some CSS comments for the "more interesting" frontend decisions

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

Very confident

Links to related issues

Any relevant links (issues, documentation, slack discussions).

• Follow-up for !1650 (merged)
• &22 (closed)

How to test

In any region or working group:

• write some new forum threads, or use the existing ones (use different accounts to test avatar handling)
• write replies to some of them (use different accounts to test avatar handling)
• check the list of forum threads
• test the layout in different resolutions (screen widths)

Screenshots (if applicable)

Thread list, default view:

Thread list, small-width view:

Previously (old responsive thread list):

Mobile post view:

Previously (old responsive post view):

Checklist

• no test: just a few frontend modifications
• 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
• 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.