Remove ghost.js

This resolves #1575.

*  Moves most code that was in `ghost.js` into `./core/server/index.js`
*  Creates `./core/server/config/theme.js` to hold all theme configurations (which previously lived on `ghost.blogGlobals()`)
*  Removed `ghost.server`, passing it in as an argument where needed and allowing middleware to hold onto a reference for lazy use.