Add an option to only allow super users to create organisations

[dabeeeenster]

Expose the superuser django User boolean field in the User API as a read only field.

Add an env var on the API called ONLY_SUPERUSERS_CAN_CREATE_ORGANISATIONS that defaults to False.

If this env var is True, only show the Create Organisation button to superuser Django admins.

[gagantrivedi]

Adds is_superuser to /api/v1/auth/users/me/ endpoint that can be used to check if the current user is super user or not