Possible Feature: Privacy levels for networks

[hmc-cs-jbearer]

Could be chosen at creation and possibly changed later by an admin. I can imagine three levels:

• Open -- Users can add themselves, and admins can invite users (the user would get an email asking them to accept the invitation)
• Private -- Admins can invite users, and users can request permission to join, pending admin approval
• Invisible -- Admins can invite users

EDIT: we could maybe handle invitations via some kind of token exchange, a la #8

[jbearer]

We can accomplish this by keeping lists of invited users, and of users who request permission to join. Admins can add users to the invite list, and users can add themselves to the network iff they are on the invite list. Similarly, users can add themselves to the request list, and admins can add the users to the network iff the users are on the request list. Open networks have no lists, private networks have both, and invisible networks have only an invite list.

We can also have the backend firebase client watch for changes in the request list and email the admin when a new user requests permission to join. We can do the same for the invite list.

Security rules now allow this, just need to add it to the front end.