Use requirejs to bundle/minify extensions, copy only what we need to …

…dist/, part of mozilla#634 (mozilla#637)

* Rework extension loading to use a json file, drop ?enableExtensions support

* Close, but no cigar

* Working, all extensions built with requirejs, copied as small as possible

* Revert change to swPrecache I removed for debugging

* Don't copy unused SourceCodePro fonts to dist/styles/fonts

src/utils/BrambleExtensionLoader.js

@@ -18,6 +18,7 @@ define(function (require, exports, module) {
 
     // Disable any extensions we found on the query string's ?disableExtensions= param
     function _processDefaults(disableExtensions) {
+<<<<<<< HEAD
         disableExtensions = disableExtensions ? disableExtensions.trim().split(/\s*,\s*/) : [];
 
         var brambleExtensions = [];