Final deprecation for constant BADGE_BACKPACKURL (it has been removed)
and badges/backpack.js file (it has been removed too).
Besides, some methods have started the deprecation process too:
 - badges_check_backpack_accessibility
 - badges_setup_backpack_js()
 - badges_local_backpack_js()

Apart from that, string 'addbackpack' has been deprecated here
because it's not used anywhere.

The $CFG->badges_site_backpack setting has been completely removed
because it's not required anymore. From now, the primary site
backpack will be the first one in the "Manage backpacks" list (so,
the one with lower sortorder value).

Mozilla backpack closed a few months ago. It was moved to Badgr.io
(as OBv1.0) but as we're also supporting Badgr.io OBv2.0, it
makes no sense to leave old references to Mozilla backpack there.

This commit combines several changes:
* Show authentication errors.
* Show more detailed authentication errors
* When the site backpack is different to the user backpack, show a warning
* Do not duplicate the apiversion and backpackurls in each user backpack.
* Do not support different issuer information for each badge with Open Badges v2.
* Add obversion to assertion url
* Combine separate badges upgrade steps and bump the version number.
* Don't show private collections.
* Push the larger image size to backpacks (512x512).
* BADGE_BACKPACKURL is deprecated
* Don't use property alignments, only alignment