Make message loading for 'ext.visualEditor.viewPageTarget.init' module less hacky

Rather than modifying $wgResourceModules at run-time, use
a subclass of ResourceLoaderFileModule to add the messages
in the constructor.

Change-Id: I9cf99891ed186afb6aa63501edced692117a7c50
Merge "Make message loading for 'ext.visualEditor.viewPageTarget.init' module less hacky"