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

Note: I had to add a new notice type to the database. Here is the command:

<code>
notification.create_notice_type("profiles_member_join_request", _("New membership request"), _("you have a new membership request"), default=1)
</code>

How to test:
   - Login with account #1
   - Create a team
   - Login with account #2
   - Visit the team page, there has to be a "Join Team" button. After clicking the button, the page gets refreshed.
   - Login with account #1
   - As the team captain you would receive a notification telling about a new request. There is a link to the membership edit page, where you have an ability to approve or reject the membership object.

   - The admin panel also lists all the membership requests where the system admin has an ability to either approve or reject it.