Move about:support add-on to AMO.

* URL for the add-on is now a setting, to change it easily.
* AMO will provide automatic updates to the add-on. We could have done
  this in SUMO, but it is not trivial.
* The version that this points to is updated to work with -dev, -stage,
  and localhost:8000.
* Remove the old add-on from the media directory.
* Add docs about the add-on.

r?