Create config module to standardise getting paths and abs URLs

This PR is for issue #1390.  Currently a WIP, with one tasks remaining: fix broken tests.  Rest of code is complete.

I followed the lead of rails (refer to the example app's https://github.com/RailsApps/learn-rails) and created a `config` directory where all configurations should reside.

This involved:

 * Moving `./core/config-loader.js` to `./core/server/config/loader.js` pretty much in its entirety.   I removed the `loadConfig` function and made it directly exported from the file as that is all that the file does.

 * Creating the `./core/server/config/paths.js`.  Most of this file's contents comes from ghost.js' `ghost.paths()` and `ghost.getPaths()`.  Those were moved into this new file and modified to not depend on the ghost singleton.

* Also moved `ghost.config()` directly to `config()`

This change to the config directory has had the lovely side effect of reducing the dependency on the ghost singleton object.

I have yet to go through the code base and update all string references to paths, however I did go through and update all `ghost.paths()` references.  That task can be a nice easy follow-up once this PR is merged.