Dashboard layout followup: CSS tweaks to chat popup, event avatars

Best review each commit individually!

What does this MR do?

Some small and tiny adjustments to CSS, as well as removal of a few rules.

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

if anything, it only breaks visuals/design. I did not test all possible resolutions in all browsers, but it should be okay.

Links to related issues

This is a part of &22 (closed) and a finishing followup to !1494 (merged) to delay the potentially breaking changes until after 🍊 release.

How to test

• begin writing very long chat messages and check the textbox behavior
• look around the platform if anything seems out of place or mis-colored
• check the event attendee details layout in various browsers

Screenshots (if applicable)

event view before:

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))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings

Again, please review each commit individually for context!