(bug 36939) Introduce a way to limit username filter to certain sources.

* Name all sources and track them inside the entries. This should be
  fully backwards-compatible with previous configuration format,
  since numbers may be a legitimate source names as well.
* Introduce $wgTitleBlacklistUsernameSources as a variable which allows
  to specify which sources may be used for filtering usernames.

Change-Id: Ia2702370c85f317c763e345c8f92cb38e23e4196