PATCH support to modify users from HTTP

Nothing complicated here, but we find it very convenient to be able to update just one property of a user document, instead of having to send it completely via a POST request.
I'd also prefer using `try (wh_util:to_atom(Module, 'true')):get_ringback(Call) of` in case Module can't be converted to an atom. The `catch` would then return `{Flow, 'undefined'}`