Can define lessons in their own github repos

[JoshCheek]

This work is mostly being done in the lessons-in-manifest branch, and is largely superseded by #34

We will need

• [ ] A manifest to tell it what endpoints to map to what repos (doesn't need to be dynamic yet)
• [x] It fetches the repo if the repo DNE
• [ ] It checks out the specified commit
• [ ] if the commit is actually a branch, it goes to that branch
• [ ] manifest tells it what file to serve
• [ ] manifest tells it what code samples to include (rather than writing them inside div tags)
• [ ] manifest tells it what test suite to use for each solution
• [ ] Include the sha in the headers

[JoshCheek]

Closing this, it is superseded by #34