Private group conversations

[Mariamcm90]

In addition to direct messages, it might be good to have private group conversations. For a student-geared chat app, this could be useful in allowing groups of students or professors and their students to have a private space to converse. This goes with #84 .

Users can be added to these conversations via email, by the conversation owner or a group member manually, or any other way you think is best!

Other things to think about include:

1. Whether or not these conversations will be visible to all chat users
2. If they are, should non-invited users be able to request access to private conversations?
3. How would these requests be displayed and who would be able to approve of them?