CSV previewing

This is WIP code that will allow useful viewing and diffing of CSV files within Gitlab. It uses [coopyhx](https://github.com/paulfitz/coopyhx) to show the diffs.

Parts that still need work:

* [ ] use same viewer for both file and diff view, for consistency
* [ ] make Blob#to_csv more robust to different CSV variants
* [ ] move Blob extensions into gitlab_git and github/linguist gems