DAR-36: track onMouseDown instead of onClick

This will prevent losing click events that don't pass data.href. Also,
this is a much better solution to the problem than using an artificial
delay of 100ms. For more information on this problem, see:
https://internal.wikia-inc.com/wiki/ClickTracking/Multiple_inputs
DAR-36: prevent tracking right/middle click.
DAR-36: catch hitting 'enter' in summary textarea to submit edits.
DAR-36: remove quick fix for tracker.stub
DAR-36: refactor Tracker plugin, update PageControls.
DAR-36: simplify categorization.
DAR-36: remove unused tracking calls.
DAR-36: send VET tracking to GA also.
DAR-36: add WikiaEditor.track()
DAR-36: whitespace.
DAR-36: fix editor type.
Merge branch 'DAR-36' into dev