Refactor Message Module

Closes #516 (closed) #602 (closed) #624 (closed) #515 (closed) #713 (closed) #721 (closed)
Blocks #413 (closed)

What does this MR do?

Refactors/Rewrites the message module to

• Gateway
• API instead of XHR
• functional refinement: Allow people to leave chats, deleting a chat when the last person left
• proper escaping

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

I am sure it might break something :-)

Links to related issues

? Related #602 (closed)

Todos

• Move sql migration filename to correct/current date

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))