3) Feature - Database Redesign

[klavenjones]

Currently, our app only allows conversations between two users. We would like to allow conversations with groups of 3 or more users. This ticket is not to implement this full change, but to help us plan out what that change would look like. Please make a_ _PR with the changes to just the database models that you would use to implement this feature. Please include "draft" in the title of the PR. Then, in the PR description, write out notes to answer the following two questions:

1. What additional changes are needed to implement this feature (please keep this as a high level summary)
2. If our app were already deployed, what steps would we need to take to move to this new feature without disrupting service for current users