Add team functionality

[OmeletWithoutEgg]

Allows users to create teams. After creating a fresh new team, an invitation link will be shown in the edit page. The invitation link contains a secret token, so anyone with that link would be able to join.

Users can join multiple teams. During the registration phase of a contest, one can choose to register as a team. The teammate's submission will then become visible in that contest. Note that in this setting, all the team members will need to manually register as that team in order to become teammates.

TODO:

• allow renewing token?
• allow admin to disable team registration.
• restrict maximum number of members in a team
• display the avatar in the registration management page
• in /teams/, somehow mark or filter teams that contains current_user
• write tests