Use namespaces when storing layers

[cmc333333]

This adds a doc_type field to layers, currently consisting of two values, 'cfr' and 'preamble' (preventing many potential collisions). This also modifies the URL pattern, which used to be /layer/[layername]/[regulation_label_id]/[version_id] to /layer/[layername]/[doc_type]/[doc_id-which-may-include-slashes] For backwards compatibility, this adds a translation layer between the old format and the new. It does, however, migrate existing data.

For eregs/notice-and-comment#17

[coveralls]

Coverage increased (+0.1%) to 87.824% when pulling 87bd9c18705a88de8eef3b79066c767d71df776b on cmc333333:17-more-layer-tweaks into 45284ea04f94372b4e3b75262b723cb599894be2 on 18F:master.

[jmcarp]

By the way, is it preferable to keep version and label in the same field? I'm wondering if we could have separate columns (with version being nullable) and avoid some confusion (on my part) about building and parsing doc_id strings.

[cmc333333]

Re splitting out version -- I was originally hoping that we could basically ignore it and treat the combined doc_id like an opaque identifier. Unfortunately, too much other stuff depends on it ATM.

How about I add a comment around that topic and kick the can down the road?

[coveralls]

Coverage increased (+0.2%) to 87.854% when pulling 1ff415396c98d48c469f62309048f325120b06f6 on cmc333333:17-more-layer-tweaks into 45284ea04f94372b4e3b75262b723cb599894be2 on 18F:master.

[jmcarp]

SGTM--I only asked because I was uncertain about the doc_id format, so I think a comment around that is fine.

[cmc333333]

Thanks for all the feedback @jmcarp ; ready for another round

[jmcarp]

:tada: