Add test to callback_url field to ensure it accepts port number

[djanderson]

Currently this implementation of callback_url does accept a port number (and forwards to the correct port, tested by nc -l 10000 and then sending to that. Note that 127.0.0.1 and localhost do not work since they have different meanings from inside the docker container but using my computer's actual IP, it forwarded to the correct port).

It should generally be acceptable, but specifically for a sample streaming action the receive application will most likely not be running on :80.

[djanderson]

Also when passing a raw ip to callback_url (e.g., 10.0.0.20:8000) the input parser rejects with a rather unuseful error "Enter a valid URL". The problem is it needs http or such in front, so either a hint in the error message or an example URL would be useful.