Flask: provide callback list and match endpoint

[fluffy-critter]

Add a callback-related endpoint _cb/check/<url>, which checks the URL and returns the pertinent info from Authl.get_handler_for_url(url), providing the name and URL schema in a JSON blob like:

{
    'name': 'Email',
    'schema': '%',
    'placeholder': 'user@example.com'
}

or the like.

If no matching endpoint exists, it should return null or undefined or false or something.