Move Helpers.docs into constants.js

This is a house cleaning ticket. It does a couple of things:
1. Moves the list of URLs stored in Helpers.docs into constants.js
2. Refactors constants to group the (now growing) list into appropriate
groups.

Now there are a lot less hardcoded "magic" strings throughout our
code, yay!

Closes COUCHDB-2474
What about keeping the `"all_dbs"` and then in the api url it matches that text string to a `constant.DOC_URLS`. Maybe just loop through the DOCS_URLS and match the key.