Implement in-app communications between users

Refer to this [discussion](https://github.com/orgs/CulverLab/discussions/29) for background information.

Implement an in-app way for users of the system to communicate with each other.

Start with the linked discussion #29 for the background and requirements behind issue #85. No files, entry points, tests, or acceptance criteria are identified, so the communication scope and what counts as done need to be established before implementation.

Written by the indexing model from the issue text.