Add new messaging preferences per users allowing them to filter who can message them

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

Before MDL-63279  gets integrated, you can add manually the $CFG->messagingallusers to your config.php for testing it.

Setup

1. Login as admin
2. Enable 'Enable users to message anyone' ($CFG->messagingallusers) in  'Site Administration | Advanced Features' 
3. Create 1 new course ('Course 1')
4. Create 3 new users (user1, user2 and user3) and enrol them on the 'Course 1'
5. Login as user1
6. Access to the user3 profile page (from the course participants page) and add him as a contact
7. Login as user3
8. Approve the contact request from user1
9. Login as admin
10. Unenrol user3 from 'Course 1'

Testing scenario 1: $CFG->messagingallusers enabled

1. Login as user1
2. Access to Participants list of the 'Course 1' and click over user2
3. Send him a message
   • CHECK the message is sent
4. Select user3 from Contacts tab in Messages
5. Send her a message
   • CHECK the message is sent
6. Login as user2
7. Go to 'Message preferences' from the user profile preferences
   • CHECK a radio button is displayed instead of the 'Prevent non-contacts from messaging me' checkbox
   • CHECK the option 'Anyone on the site' is displayed and checked.
8. Set the messaging preference to ' My contacts only'
9. Login as user1
10. Access to Participants list of the 'Course 1' and click over user2
11. Send him a message
    • CHECK the message is NOT sent. An error message with the cause will be displayed (something like 'You can\'t message {$a} due to their message preferences. Try adding them as a contact.')

Testing scenario 2: $CFG->messagingallusers disabled

1. Login as admin
2. Disable 'Enable users to message anyone' ($CFG->messagingallusers) in  'Site Administration | Advanced Features' 
3. Login as user3
4. Go to 'Message preferences' from the user profile preferences
   • CHECK the option 'Anyone on the site' is NOT displayed
   • CHECK 'My contacts and anyone in my courses' is the selected option
5. Login as user1
6. Access to Contacts tab in Messages and select user3
7. Send her a message
   • CHECK the message is sent
8. Access to Participants list of the 'Course 1' and click over user2
9. Send him a message
   • CHECK the message is NOT sent. An error message with the cause will be displayed (something like 'You can\'t message {$a} due to their message preferences. Try adding them as a contact.')
10. Login as user2
11. Go to 'Message preferences' from the user profile preferences
12. Set the messaging preference to ' My contacts and anyone in my courses'
13. Login as user1
14. Access to Participants list of the 'Course 1' and click over user2
15. Send him a message
    • CHECK the message now is sent