MDL-44725 Availability: Replace groupmembersonly - core_availability (2)

Remove groupmembersonly usage in the core_availability API, and change
the update code (used in backup) so that it considers groupmembersonly
when restoring old backups.
MDL-44725 Availability: Replace groupmembersonly - backup (3)

Change backup and restore to not incldue the field, and to cope
correctly with 2.6 (and before) and 2.7 backups.
MDL-44725 Availability: Replace groupmembersonly - forms (4)

Remove groupmembersonly option from module editing forms.
More core changes
MDL-44725 Availability: Replace groupmembersonly - login (5)

Remove groupmembersonly option from require_login.

NOTE: This code, and another branch (both related to login when
accessing activities on the site course), are not actually
necessary because there is code above which already checks
$cm->uservisible.

NOTE: You can see this change more simply by doing
git diff --ignore-space-change; indent was altered.
MDL-44725 Availability: Replace groupmembersonly - FEATURE_xx (6)

Remove old FEATURE_GROUPMEMBERSONLY as no longer required.
Assig module changes
Glossary module changes
Quiz changes
MDL-44725 Availability: Add SQL feature for user lists (N)

The previous API included a facility to filter a list of users
to include only those who are allowed to access an activity, i.e.
only people who belong to the required groups etc.

This change adds a new API function to return SQL that obtains
this list of users, so that it can be combined with other
queries.