Feature/join team button

- Added ability for users to join teams when the tournament is in signup phase
      * Made sure that already members cannot join to teams
      * Made sure that a user cannot request more than one membership
- Sending on-site notifications to the team captains, once a new awaiting request has been created,
- Added ability for team captains to approve or reject the team membership
- Added ability to site admins to review/approve/reject requested team memberships