User groups / organizers

[plexus]

A person who creates a session should be able to edit it.

Orga folks should be able to edit any session.

It should be possible to add other organizers to your session, so both (all) of you can edit it and access organizer tools.

We currently have :session/organized, which is intended to be a ref to a user, I think it would be good to introduce a user-group entity as well, and let :session/organized be either a user or user group. We can then implement access control on user and group level.

[humorless]

In newest access control of the Edit button, I implemented three conditions to let the Edit button be active:

1. The user is the organizer.
2. The organizer records a user group, and the user belongs to that user group.
3. The user belongs to the orga user group.

For condition 3: I create a file inside repl-sessions/user_group.clj to create a orga user-group and add user eid into it.

For condition 2, I am thinking that probably I should create an INVITE Organizer UI control. Once the activity organizer inviting anyone, the session's organizer will point to a new user group which is created automatically.