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 that would totally work. This is such a subtle point, but I guess I'd argue that it's clearer with the long constants name. If the `documentation` method was `docUrl` or something to clearly indicate that we're returning a documentation *URL*, I'd be more in favour of returning the `all_dbs` string, because you could infer what it was just from looking at the code. However, you still wouldn't know where to find the URL. At least with the long constant it's immediately clear where the info's stored.

But I'm okay with your suggestion. So it would return the uppercase constant name?