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/resolvers/DefaultResolver.ts

@@ -0,0 +1,34 @@
+import Mithril from 'mithril';
+
+/**
+ * Generates a route resolver for a given component.
+ * In addition to regular route resolver functionality:
+ * - It provide the current route name as an attr
+ * - It sets a key on the component so a rerender will be triggered on route change.
+ */
+export default class DefaultResolver {
+  component: Mithril.Component;
+  routeName: string;
+
+  constructor(component, routeName) {
+    this.component = component;
+    this.routeName = routeName;
+  }
+
+  /**
+   * When a route change results in a changed key, a full page
+   * rerender occurs. This method can be overriden in subclasses
+   * to prevent rerenders on some route changes.
+   */
+  makeKey() {
+    return this.routeName + JSON.stringify(m.route.param());
+  }
+
+  onmatch(args, requestedPath, route) {
+    return this.component;
+  }
+
+  render(vnode) {
+    return [{ ...vnode, routeName: this.routeName, key: this.makeKey() }];
+  }
+}