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
Ok, you make a valid point. Lets leave the FauxtonAPI.constants in then. Returning just the `"all_docs"` does add a lot of magic which is probably unnecessary.