Mithril 2 update (#2255)

* Update frontend to Mithril 2

- Update Mithril version to v2.0.4
- Add Typescript typings for Mithril
- Rename "props" to "attrs"; "initProps" to "initAttrs"; "m.prop" to "m.stream"; "m.withAttr" to "utils/withAttr".
- Use Mithril 2's new lifecycle hooks
- SubtreeRetainer has been rewritten to be more useful for the new system
- Utils for forcing page re-initializations have been added (force attr in links, setRouteWithForcedRefresh util)
- Other mechanical changes, following the upgrade guide
- Remove some of the custom stuff in our Component base class
- Introduce "fragments" for non-components that control their own DOM
- Remove Mithril patches, introduce a few new ones (route attrs in <a>; 
- Redesign AlertManagerState `show` with 3 overloads: `show(children)`, `show(attrs, children)`, `show(componentClass, attrs, children)`
- The `affixedSidebar` util has been replaced with an `AffixedSidebar` component

Challenges:
- `children` and `tag` are now reserved, and can not be used as attr names
- Behavior of links to current page changed in Mithril. If moving to a page that is handled by the same component, the page component WILL NOT be re-initialized by default. Additional code to keep track of the current url is needed (See IndexPage, DiscussionPage, and UserPage for examples)
- Native Promise rejections are shown on console when not handled
- Instances of components can no longer be stored. The state pattern should be used instead.

Refs #1821.

Co-authored-by: Alexander Skvortsov <sasha.skvortsov109@gmail.com>
Co-authored-by: Matthew Kilgore <tankerkiller125@gmail.com>
Co-authored-by: Franz Liedke <franz@develophp.org>

js/src/common/components/Page.js

@@ -7,10 +7,14 @@ import PageState from '../states/PageState';
  * @abstract
  */
 export default class Page extends Component {
-  init() {
+  oninit(vnode) {
+    super.oninit(vnode);
+
     app.previous = app.current;
     app.current = new PageState(this.constructor);
 
+    this.onNewRoute();
+
     app.drawer.hide();
     app.modal.close();
 
@@ -22,13 +26,27 @@ export default class Page extends Component {
     this.bodyClass = '';
   }
 
-  config(isInitialized, context) {
-    if (isInitialized) return;
+  /**
+   * A collections of actions to run when the route changes.
+   * This is extracted here, and not hardcoded in oninit, as oninit is not called
+   * when a different route is handled by the same component, but we still need to
+   * adjust the current route name.
+   */
+  onNewRoute() {
+    app.current.set('routeName', this.attrs.routeName);
+  }
+
+  oncreate(vnode) {
+    super.oncreate(vnode);
 
     if (this.bodyClass) {
       $('#app').addClass(this.bodyClass);
+    }
+  }
 
-      context.onunload = () => $('#app').removeClass(this.bodyClass);
+  onremove() {
+    if (this.bodyClass) {
+      $('#app').removeClass(this.bodyClass);
     }
   }
 }