MDL-43369 : quiz statistics - average across attempts

where item seen and not across all attempts for subqs.
MDL-43440 unit testing: Use cmid in unit tests when it should be used.

instance is has been used instead of cmid.  This works as all id's are 1
in most cases, but not always.

accesslib_clear_all_caches_for_unit_testing was updated to clear the
$USER->access cache which would fail test when the wrong user's data was used.
MDL-39942: invite only users who can post to discuss
MDL-43884 database: Fix get_tables() api for external stores
MDL-43892 dbtransfer: Fix missing string.
MDL-43805 assign: Quickgrading wipes out grades for hidden columns.

Grr.. quickgrading will always be a hack.

Conflicts:
	mod/assign/locallib.php
MDL-43805 assign: Behat test for quickgrading bug
MDL-37714: Correctly identify frequently used comments during restore
MDL-33920 filepicker: hide 'save as' field when not applicable
MDL-41384 upgrade: make sure to re-read $SITE from DB when course table is altered
MDL-41384 course: do not show debugging message if $COURSE contains format options

before showing debugging message make sure that field exists in database and not only in the object
MDL-43821 completion: fix bug in section completion progress

Thanks to Shane Dye for the provided patch
MDL-43828 forum: allow to select single available group when posting
MDL-41647 WebServices: Changed notification CSS class
Merge branch 'MDL-43440-25' of git://github.com/mr-russ/moodle into MOODLE_25_STABLE
Merge branch 'MDL-43805-25' of git://github.com/damyon/moodle into MOODLE_25_STABLE
Merge branch 'MDL-41647_25' of git://github.com/apsdehal/moodle into MOODLE_25_STABLE
Merge branch 'm25_MDL-37714' of git://github.com/greg-or/moodle into MOODLE_25_STABLE
Merge branch 'wip-MDL-39942-MOODLE_25_STABLE' of https://github.com/kwiliarty/moodle into MOODLE_25_STABLE
Merge branch 'MDL-43884-25' of git://github.com/ankitagarwal/moodle into MOODLE_25_STABLE