Add csvm handlers for release & revisions

[andrewmcveigh]

Closes: #219

This PR implements (empty) csv metadata as per the spec[1], by:

• Adding a Link header to csvs for releases and revisions, to point to the json metadata file for the csv. (at {csv-url}-metadata.json)
• Adding a placeholder json metadata file at the url {csv-url}-metadata.json, where a resource at {csv-url} exists.
• Adding a metadata URI pattern spec at /.well-known/csvm. (though according to spec this is redundant)

[1] https://www.w3.org/TR/2015/REC-tabular-data-model-20151217/#locating-metadata

[andrewmcveigh]

Ah, yes. I meant to do that :pray: