* Add focus ring mixin
These mixins allow us to restore default browser focus rings on elements which no longer have them.
* Add info about custom outline styles; use `#private` namespace and fix mixin name
I just learned that Less has namespaces! https://lesscss.org/features/#mixins-feature-namespaces
* Replace gulp with webpack and npm scripts for JS compilation
* Set up Travis CI to commit compiled JS
* Restructure `js` directory; only one instance of npm, forum/admin are "submodules"
* Refactor JS initializers into Application subclasses
* Maintain partial compatibility API (importing from absolute paths) for extensions
* Remove minification responsibility from PHP asset compiler
* Restructure `less` directory
