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
Yeah, this was my original concern with having to type out `FauxtonAPI.constants` instead of just `C` or something. It got verbose awfully fast. 

I don't have another solution other than storing the var in a local var to reduce the length, or maybe renaming the main constants var to `FauxtonAPI.C`. Meh. :(

But personally I don't think this is all that bad. It's certainly verbose but clear.