Add National Squad admin interface

[davidsandoz]

Currently, the National Squad is managed on Podio. However, this means that we have a duplicate of the members on tchoukball.ch and on Podio, which is not efficient.

It would therefore be much better to manage everything directly from tchoukball.ch

This would also allow the players to connect with their own account and manage their subscription directly on tchoukball.ch.