Minimal Interesting Binder

Having a binder that does... anything... would be really useful.

Once #29 is good to go, I'll be happy to do the work to add a read-only `AnonGitHubManager` which would let you at least _look_ at something (until you ran out of requests).

Some deltas here:
- this manager would require a starting point (e.g. a specific org or repo)
- but we probably want that _anyway_
- carefully managing n+1 queries (don't list files/discussions until needed)
- probably need to do _that_ anyway
- add a Router endpoint that avoids many queries and gets you to the good stuff

From a re-use perspective, if all a project had to do was...
```
# requirements.txt
jupyterlab-pullrequests
# diff-match-patch
```

And then a binder URL so that, e.g. `jupyterlab/jupyterlab` PR 1234 was:

```http
GET https://mybinder.org/v2/gh/jupyterlab/jupyterlab/HEAD?urlpath=lab/prs/agh/jupyterlab/jupyterlab/1234
```

...to bop you right into the PR overview. That could then happily grab the list of files and the discussion, and you would most likely be able to view a lot of what you needed before you hit the budget.