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/forum/components/DiscussionList.js

@@ -7,17 +7,13 @@ import Placeholder from '../../common/components/Placeholder';
 /**
  * The `DiscussionList` component displays a list of discussions.
  *
- * ### Props
+ * ### Attrs
  *
  * - `state` A DiscussionListState object that represents the discussion lists's state.
  */
 export default class DiscussionList extends Component {
-  init() {
-    this.state = this.props.state;
-  }
-
   view() {
-    const state = this.state;
+    const state = this.attrs.state;
 
     const params = state.getParams();
     let loading;
@@ -25,11 +21,13 @@ export default class DiscussionList extends Component {
     if (state.isLoading()) {
       loading = LoadingIndicator.component();
     } else if (state.moreResults) {
-      loading = Button.component({
-        children: app.translator.trans('core.forum.discussion_list.load_more_button'),
-        className: 'Button',
-        onclick: state.loadMore.bind(state),
-      });
+      loading = Button.component(
+        {
+          className: 'Button',
+          onclick: state.loadMore.bind(state),
+        },
+        app.translator.trans('core.forum.discussion_list.load_more_button')
+      );
     }
 
     if (state.empty()) {