Improve minister select widget

This branch creates a new auto complete minister select jQuery plugin. It's based on a jQuery autocomplete plugin, with added support to work on existing <select> fields. The idea is that in the future this ministerSelect plugin could be extracted and used within MoJ / GDS for internal systems. But it's not needed at the moment. 

Second part of the branch cleans out our assets folder a bit by
* using vendor assets from the respective gems
* moving assets to `vendor/assets` folder
* only overriding changed css rules
* fixing the require order in application.{js,css}