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
I like the idea of the docs being in `constants.js` but I don't like that we have to go from `"all_dbs"` to `FauxtonAPI.constants.DOC_URLS.ALL_DBS` everytime we want a documentation string. Could we not have a way of mapping the doc strings so that we don't have to do that?