Release 3.1.0 for Moodle 3.1 and later

Initial release of the new branch for Moodle 3.1 and later.

Major feature changes:

• Confidential appointment notes (visible to teacher only) can now be added.
• Students can be allowed to see existing bookings of other students.
• Invitations are now sent trough the web interface rather than via a "mailto:" link