Updated private_message from 1.0 to 1.2

Difference between the versions are 6 bug fixes

1. 1.1 #2974254: Emails being sent to users who have disabled notifications
2. 1.1 #2971692-7: Email notifications contain HTML tags
3. The patch which was included composer.json is removed since it committed and fixed in in 1.1 #2972948: Entity field weights
4. 1.2 #2976382: Make access more permissive
5. 1.2 #2976378: Make members list prefix configurable throug the UI
6. 1.2 #2976552: Fatal error when rendering an empty thread