Tidy Gruntfile redux

 * removing outbrain and gravity
 * only copy across the 3rd-party lib files we actually use
 * simplify again by removing ability to compile a particular app's js, e.g. `compile:js:facia` (was anyone using this?) - devs should be using `watch:js`
Above means if a new lib is added, the file to copy across will need to be added here