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 url used here will automatically point to the latest version.
* Remove the old add-on from the media directory.t
* Add docs about the add-on.
* The version of the add-on in AMO will work on -dev and -stage.