Enable "support" users #57
Description
Sometimes, users will need to invite a third party (e.g., a consultant, or a BDK support person) to help them use the tool. For example, if a bdk.cs.vt.edu user needs help, we should be able to log in and see what they see.
One way to do this would be to allow certain users to be "admins" who can see any account on a server. This can lead to tricky security bugs though. Another would be to build this on top of our organization feature, and allow users to join multiple organizations. That way, a support person could be explicitly invited to an organization (just like any other user) to help someone out temporarily, without building new code paths.