Chat UI improvements

What does this MR do?

improved layout/UI for chatbox and conversation tested on mobile and screen. please feedback for colours. Logged users messages are classed "mymessage" and can be accessed and styled separately. added notification badge.

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

Should work. Only notification dropdown requires beta testing. But I'm confident!

Screenshots

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• asked someone for a UI review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))