Add simple method to check if the provided user is an admin of the given Site
Create the @login_required decorator
Specify in the main controller which endpoints require a login
Add a generic method to check is the current user is authenticated
Add an @admin_required decorator
You must be an admin to view all sites
Make the @login_required decorator use the is_authenticated method
Add method to retrieve a SiteAdmin via its identifier
Add endpoint to add an admin to an existing site
Add endpoint to delete an admin from a given endpoint
Adjust the Site view to fix the link/layout to list admins and hosts and add/delete admins
Small pep8 fix