Minor dashboard redesign (activity update layout + colors)

What does this MR do?

Very small design and layout adjustments to activity updates on dashboard:

• better color contrast
• more bold elements in low-contrast situations
• truncate posts later (display longer snippet)
• improve post structure and quickreply position to avoid some linebreaks
• small CSS refactoring included (just getting rid of duplicate definition)

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

I'm confident it's improving the overall layout!

How to test

Login as local testuser with dashboard updates and check the different types, quickreply etc in various screen widths and with short/long content

Screenshots

New:

Old:

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
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo

Release notes text

Die Anzeige von Updates auf dem Dashboard ist jetzt visuell strukturierter als zuvor. Euch wird sicher der fett gedruckte Text für die Informationen in der Kopfzeile auffallen. Es gab an der Stelle Verbesserungswünsche, da der recht helle Text teils zu dünn präsentiert wurde, um noch gut lesbar zu sein. Sehr wahrscheinlich passen wir in Zukunft das Aussehen dieser Texte nochmal ein wenig an.