Take arbitrary git url

[jyn514]

Hunter gave me this idea - front end should take an arbitrary git url, like a github repo. This would not be terrible to implement on the backend, not sure how to implement it on the front end.

Ideally, this would NOT download the whole repository, but only checkout the files we want to highlight. git clone --no-checkout is the syntax.

Github has an API for finding the size of a repo - eventually we should check if the url is on github, check if the size is greater than, say, 100 MB, and if so only work remotely, don't download the whole history.

[jyn514]

We could also use git clone --depth 1 and deepen it as necessary.