[bug 969667] Merge desktop and mobile forms

* create new generic form that merges desktop and mobile forms
* move around instructions so important instructions come before the
  fields they apply to--this is better for privacy-related information
* add pushstate support so going back and forth in the form works with
  form controls and browser controls
* this doesn't support jumping into the middle of the form
* rework infrastructure so it's possible to add additional cards and
  measure drop-off rates
* makes this structurally like the firefox os feedback form so all forms
  now have a similar structure

This looks a little worse on Desktop, though. This form could definitely
use better strings and better design, but that's out of the scope of
this change and would require a lot more work.