Customize users authentication

[davidcr01]

This PR modifies how the users are authenticated and can use the API.

The API is protected against non-authenticated users, except for some endpoints to create new users and players.

• User and player creation are allowed for everyone.
• List of users is available to EM.
• List of players is available to authenticated users.
• Update of users are available to EM and owners.
• Update of players are available to EM.
• Deletion of users and players is available to EM and owners.
• Rest of operations for authenticated users.

This PR also needs https://github.com/davidcr01/TFG/issues/7 to be closed before merging.

Also, some improvements have been done:

• The is_superuser field has been changed to read_only, to avoid changing it (security reasons).
• A new logic has been added to avoid the password field of the user being updated while modifying the user information (security reasons)
• The player destruction has been overwritten to destroy also the user information (cascade deletion).
• The return of the player information has been modified to hide some private information of the related user. New serializers have been created.