Port modular javascript framework from transition

* Originally written in alphagov/transition#192
* Rename global from GOVUK to GOVUKAdmin to avoid clashes with existing
JavaScript within applications
* Using GOVUKAdmin makes it clearer when apps are using the gem’s
javascript
* Include documentation in JAVASCRIPT.md
* Include useful modules:
  * Automatically show bootstrap modals on page load
  * Automatically track Google Analytics events on page load
  * Filter the contents of a table
  * Fix a table’s header when scrolling
  * Select items in a table and perform an action on them
  * A simple toggle for hiding/showing — toggles target elements