Add uniqueness constraint to message DTO (simplifies other logic)
In the case where a message thread is tied to an entity (i.e. a comment thread), there shouldn't be any user-ID filtering.  The filtering should be on the entire thread, done on the manager level (i.e. does the user have READ permissions on the entity?)
Add link between MessageThreads and Nodes
Migration tests...
Need a conversion between threads and nodes and vice versa.
+ Manager interface