Messaging: Update the message APIs allowing teachers to send 1:1 messages to students, regardless of their messaging prefs.

It should be covered by PHPUnit tests. However, here there are also some manual tests.

This should be integrated/tested at the same time (or later) than MDL-63214.

Setup

1. Login as admin
2. Create 2 new courses
   1. Course 1
   2. Course 2
3. Create 4 new users
   1. user1
   2. user2
   3. user3
   4. teacher1
4. Enrol user1 and user2 as students on the 'Course 1'
5. Enrol teacher1 as a teacher to the 'Course 1'
6. Enrol user3 and teacher1 as students to the 'Course 2'
7. Login as user1
8. Access to the teacher1 profile page (from the course participants page) and block him as a contact following these steps:
   1. Click on the 'Message' icon near to the teacher name
   2. Click on the teacher name you'll find at the top of the message area (above the Online/Offline text)
   3. Click on the 'Block contact' link
9. Login as user2
   1. Set the messaging preference to 'My contacts only'
10. Login as user3
    1. Set the messaging preference to 'My contacts only'

Testing scenario 1: Send 1:1 message as a teacher

1. Login as teacher1
2. Navigate to Course 1 => Participants => user1
3. Click the Message link at the top of the profile
4. Send a message
   1. Confirm that the message is sent
5. Navigate to Course 1 => Participants => user2
6. Click the Message link at the top of the profile
7. Send a message
   1. Confirm that the message is sent
8. Access to Participants list of the 'Course 2' and click over user3
9. Click the Message link at the top of the profile
   1. Confirm that an error was shown

Testing scenario 2: Send bulk message as a teacher

1. Login as teacher1
2. Navigate to Course 1 => Participants
3. Select user1 and user2 and choose 'Send a message' from the 'With selected users...' list
4. Write the message and click over the button to send it
5. Login as user1
   1. Confirm that the message was received
6. Login as user2
   1. Confirm that the message was received

Test messaging unrelated users

1. Login as user1
2. From the Message icon (next to the User menu), choose "New message"
3. In the "Search for a user or course" field, search for "user3"
4. Attempt to send User 3 a message
   1. Confirm that you cannot

Test with site messaging for all users

1. Navigate to Site administration => Advanced features
2. Tick the messagingallusers setting and save changes
3. Login as user3
4. From the user menu, choose Preferences
5. Choose "Message preferences"
6. Change the value to "Anyone on this site"
7. Login as user1
8. From the Message icon (next to the User menu), choose "New message"
9. In the "Search for a user or course" field, search for "user3"
10. Attempt to send User 3 a message
    1. Confirm that can now send user a message