Version the tutorial

It would be nice to have multiple versions of the tutorial, corresponding to the major versions of `servant`. Preliminary work [here](https://github.com/haskell-servant/haskell-servant.github.io/compare/jkarni/versions?expand=1).

The way I'd propose to do this:
- [ ] Make separate branches for each version (e.g. `versions/0.4`, `versions/0.5`, etc.).
- [ ] Make a separate branch for non-tutorial changes.
- [ ] Make the `deploy.sh` script check what branch it's on.
- If one of the version branches, then copy over only the tutorial content into an appropriate version subdir (`tutorial/0.4/somecontent.html`).
- If the non-tutorial branch, only copy content outside of `tutorial` folder.
- [ ] Redirect `tutorial/somecontent.html` to `tutorial//somecontent.html`.
- [ ] Make sure links still work locally and on travis.