Allow extensions to use route resolvers (#2275)

- mapRoutes: don't wrap components in resolvers if they are already resolvers
- Extract defaultResolver into its own class
- Allow either route resolver instances, or components with an optional resolverClass which should accept the component and route name in its constructor.
- Introduce a resolver for DiscussionPage, so that routing from one post to another on the same discussion triggers a scroll instead of rerendering

js/src/common/compat.js

@@ -67,6 +67,7 @@ import username from './helpers/username';
 import userOnline from './helpers/userOnline';
 import listItems from './helpers/listItems';
 import Fragment from './Fragment';
+import DefaultResolver from './resolvers/DefaultResolver';
 
 export default {
   extend: extend,
@@ -138,4 +139,5 @@ export default {
   'helpers/username': username,
   'helpers/userOnline': userOnline,
   'helpers/listItems': listItems,
+  'resolvers/DefaultResolver': DefaultResolver,
 };