mediawiki.js: Refactor definition of mw.log singleton

mediawiki.js: Refactor definition of mw.log singleton

Move this out of the large plain object that defines 'mw' itself
because right now none of the mw object initialisation is able
to call mw.log.

In a future commit, a call to mw.log is introduced in the mw.Map()
constructor, which is also used to set the mw.messages, mw.templates
and mw.user.options fields.

* Rename the other log() function to avoid clashing.
* Update outdated comment about not using mw.log() which was
  there about confusion between log() and mw.log(). However,
  this code was updated long ago to use mw.track() instead of
  log() directly, and log() has now been renamed.

Change-Id: I7d6bd4e3c9753e30278752c59d6cbd4d2fd10d96