Fx60: Fix issues with JS code modules

- The Mozilla CommonJS loader is no longer available, so bundle the
  Fx52 version of it
- Strict mode is enforced
- `this` is only defined as a global object in .jsm files, not .js files
- `this` can't be converted to a string for BackstagePass test, so check
  for presence of Components.utils.import instead
- The return value from import() is no longer available

components/zotero-service.js

@@ -161,7 +161,7 @@ Components.classes["@mozilla.org/moz/jssubscript-loader;1"]
 	.getService(Components.interfaces.mozIJSSubScriptLoader)
 	.loadSubScript('resource://zotero/require.js');
 
-ZoteroContext = function() {}
+var ZoteroContext = function() {}
 ZoteroContext.prototype = {
 	require,