Forum post not visible to students when forum is separate groups and post is sent to all participants

Test setup

On the 27 and 28 branches only - before applying the patch (or with it reverted):

1. Create a course with 1 group in it
2. Create three forums in the course - one in each group mode
3. Post some discussions in each forum:
   1. No groups - just post
   2. Separate groups - one post to 'All participants', one post to your group
   3. Visible groups - one post to 'All participants', one post to your group
      (you should have a total of five posts)
4. Reply to each discussion
5. Inspect the forum_discussions table in the database. You should see a groupid of 0 is present.
6. As a student in the group:
   1. Confirm that you cannot see the discussions written to 'All participants'

Testing instructions

1. Run the Moodle upgrade
   1. Verify that the forum_discussions table no longer has groupid of 0. These should now all be -1
2. Post and reply in all forums and groups again
   1. Verify that the forum_discussions table has no records with a groupid of 0
3. As a student in the group:
   1. Confirm that you can see every discussion

Note: For 29/master changes, you need to take a 28 site from last week's stable, and perform the upgrade. You will need bad data before each upgrade - e.g. 28 -> 29; and then a separate 28 -> master