Add ability to join team by token

[rbclark]

Currently users have to be invited to a team or request to join a team. Add the ability for users to also join a team using a private token that the team captain can distribute. From there they can still add remove users as needed.

[noraj]

More context:

[Bialogs]

@rbclark This would be great for the eCTF as the captain can distribute the token to the students.

[rbclark]

Removing from the eCTF milestone since that has passed, however this is still something we are planning to add.