Overhaul of the AbuseFilter JS

I've changed the JS of the AbuseFilter extension to match our
coding conventions and made it use mediaWiki.Api(). Furthermore
I fixed several error handlers and made some things nicer in
general while trying to not change the user appearance.

I've tested the changes as good as I could (I'm pretty sure everything
works).

Change-Id: Iaa0f99367bd7e0bfaf7249dcc9ac9ce9881b46e1
Merge "Overhaul of the AbuseFilter JS"