Add multi-user support

[vpetersson]

We need to be able to support multiple users within the dashboard.

We need to have a few roles:

Role	Description
Owner	This is the person who created the account. There can only be one.
Billing	This permission allows access to billing settings.
Viewer	This role can only view data (e.g. not revoke or add screens)
Operator	This person can add/revoke nodes and edit data (but not billing)

User management

Only the admin should be able to add users. We need a new "Team" tabs in our profile setting where the admin can invite other users using an email address.

We also need the ability to change permission of users and to remove users.

Back-end

We need to overhaul the mapping of nodes. Currently they are tied to a user, but with this change, they need to instead be tied to the team.

[rptrchv]

Consider using https://github.com/bennylope/django-organizations for this task