MDL-46282 core: Add Inbound Message API

This issue is a part of the MDL-47194 Task.
This issue is a part of the MDL-39707 Epic.
MDL-46284 core: Add e-mail fetcher from IMAP

This issue is a part of the MDL-47194 Task.
This issue is a part of the MDL-39707 Epic.
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.