Language metric support.

- [x] Parsing file names to recognize languages.
- [x] Keeping a scoreboard for each language.
- [x] Tests.

**Front-end changes.**
- [x] New views routes and templates.
- [x] Changes to `CommitResource` to allow fetching of commits by language.
- [x] Changes to `JULY.CommitCollection` to work with the changed api.

Addresses issue https://github.com/julython/julython.org/issues/46.