EPIC: Message Centre

As a user of the system (both internal and external),
I want to view and create new correspondence in the system,
So that all communication is centralized around the file and is auditable.

**Note:**
Intention is to move away from email, phone and other comms channels, and instead to centralize all relevant correspondence in BCAP.

No files, tests, or entry points are named in the issue; begin by reviewing the repository structure and clarifying the frontend and backend areas involved in correspondence. Done means internal and external users can view and create correspondence centralized around a file, with communication auditable in BCAP.

Written by the indexing model from the issue text.