Add year to messaging conversation date headings, if not the current year

Note: testing required database access to update message dates

1. Login as any user
2. Open messaging drawer
3. Click on your self conversation under Starred
4. Send two messages: "Message one" and then "Message two"
5. Confirm short date is shown, e.g. 18 December
6. In your database editor, you should now have two records with the following query:

   SELECT * FROM moodle.m_messages;

7. Backdate the time of the first message using the following query:

   UPDATE m_messages SET timecreated = 1559913000 WHERE id = 1;

8. Reload Moodle in your browser
9. Open messaging drawer
10. Click on your self conversation under Starred
11. Confirm the date portion of the first message is "7 June 2019" (e.g. full date)
12. Confirm the date portion of the second message is "18 December" (e.g. short date)