Currently all Messengers send emails only. A system for storing and displaying messages to individual participants as well as all members of a partner should be added to the server for use by Messengers.

Use cases:
* a system message such as an upcoming change in the terms of service
* notification of publication of a posted asset shown to the user in the management application
* reminders of subscription payments

This should not replicate the features provided by the discourse forums. It would not be for participant-to-participant or participant-to-partner, for instance.

Reproducible: Always