simple gist interface to replace Open in GitHub

[gordonwoodhull]

When we use our own backends instead of GitHub, it's a big loss not to have Open in GitHub:

• it provides an objective view if we suspect that the cells are displayed wrong
• it provides a way to look at the code without logging into RCloud
• it shows the revisions all in one page, with diffs

It will be a lot of work to implement this, so I guess we have to go with #1838 for 1.8 and disable the feature for now, but maybe we could do a readonly version of the gist interface to provide the above features.