Clean up abusefilter.php

This file was a bit of a mess. Main changes are:
- update comments, remove obsolete comments
- use a switch statement instead of if,elseif,elseif,elseif,...
- put wikis in alphabetical order instead of the random
  order they were in previously

No actual code changes are being made here.

No bug.

Change-Id: I220fe9ee428c4653d1026089b70c69dbe853b9ce