Allow full URLs to support multiple repository providers

[jmunroe]

Current the form assumes that the 'GitHub Repository' will be specified as the repo name only but users may assume the full URL is required:

Could not resolve ref for gh:https://github.com/binder-examples/conda/HEAD. Double check your URL and that your repo is public. GitHub recently changed default branches from "master" to "main".

The GitHub repository binder-examples/conda works as expected.

Since the Binder API already supports multiple repository provides, let's generalize the behavior so that GitHub repos are not an explicit requirement.

[jmunroe]

@yuvipanda -- I am not an owner on this repo but could you assign me this one? I like to give a shot at fixing it. (Good practice for all of those JS/React tutorials I worked through previously but don't get much of a chance to use at the moment!)