Actually pass the addon to the form (bug 892694)

The tests from commit c72ff7f2 should cover this.
fix api test
Make manifest_url not required for packaged apps in basic form (bug 800341)

Also make cat_form go with the rest of the form to prevent changes
to be saved if a form is invalid (previously, since cat_form was
valid and it saved the addon instance, invalid changes could be
saved as long as the cat_form was valid)