Story 1516: New A/B test for Sign-up Edit Guider

If the user signs up from the left nav and they belong to the test
group, load an edit guider when they return to the article they
came from.

Also fixing the fact that edit guiders are currently broken due to the
edit button not being activated in time. Now mobile.editor is
explicitly loaded first (as a dependency).

Change-Id: Ibce39dd63bd5200a4d68b34efcf3a3a2abd09d28
Merge "Story 1516: New A/B test for Sign-up Edit Guider"