Tabulated data

[joeroe]

My proposed basis for implementing #115

• Adds csv as a content type that controllers can respond with
• Adds ability to export records as CSV
  • Uses https://github.com/diogob/postgres-copy, which is (very!) fiast
  • Uses a pseudo-templating system to define which columns are exported to CSV and how (see e.g. app/views/c14s/index.csv)
• Adds csv format to index actions of all data records
• Adds buttons for downloading CSVs to index views of sites, c14s, typos, and references
• Adds buttons for downloading CSVs of associated records from the C14 show view
  • Sort of a proof of concept... for this to be extended to all views, we need a more consistently implemented filtering API (related: v2 API #147, Filtering API is missing a frontend #211)

This is a different approach from our old one, of collating all models together into one. It delegates the responsibility of reconstructing our data model and flattening it into one table to the end user (if that's what they want to do). I think this is the right approach. Our data model isn't flat and trying to pretend it is tends to cause problems (e.g. #165)

That said, I do think it would be useful to pull in fields from associated records when there is a straightforward one-to-one relation (#255). Putting the site name in the c14 table, for example.

Other potential improvements/extensions:

• Provide other formats (TSV, XLSX), Provide tabulated indexes in other formats #256
• Make CSV downloads available in the data browser, Make tabulated downloads available in the data browser #257