Handle other sources than github.com

[jtpio]

Just like mybinder.org (and other BinderHubs). the gallery website should also handle other sources than github.com.

This is an issue for the following TODO:

voila-gallery.github.io/js/index.js

Lines 10 to 12 in fb85561

	// strip github.com from the repo url
	// TODO: handle other sources
	repoUrl = repoUrl.replace("https://github.com/", "");

For reference, the BinderHub frontend lives here: https://github.com/jupyterhub/binderhub/tree/23eeb4313d96890e3cc777e614135fa224da868c/binderhub/static/js