Release 3.1.2 for Moodle 3.1 and 3.2

This is a maintenance release with few feature changes.

Feature change: Optionally, block individual bookings when group scheduling is used (new global config option)

Bug fixes: CONTRIB-6873 (teachers sometimes see empty slot table), CONTRIB-6823 (Oracle compatibility), CONTRIB-6891 (show other students in "Upcoming slots" table), among others.