MDL-3714 mod_forum: Add an inbound message handler

This issue is a part of the MDL-47194 Task.
This issue is a part of the MDL-39707 Epic.
MDL-47197 core: Private files handler

This issue is a part of the MDL-47194 Task.
This issue is a part of the MDL-39707 Epic.
Merge branch 'MDL-47197-master' of git://github.com/andrewnicols/moodle into master-proposed