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/PostEdited.js

@@ -6,18 +6,20 @@ import extractText from '../../common/utils/extractText';
  * The `PostEdited` component displays information about when and by whom a post
  * was edited.
  *
- * ### Props
+ * ### Attrs
  *
  * - `post`
  */
 export default class PostEdited extends Component {
-  init() {
+  oninit(vnode) {
+    super.oninit(vnode);
+
     this.shouldUpdateTooltip = false;
     this.oldEditedInfo = null;
   }
 
   view() {
-    const post = this.props.post;
+    const post = this.attrs.post;
     const editedUser = post.editedUser();
     const editedInfo = extractText(app.translator.trans('core.forum.post.edited_tooltip', { user: editedUser, ago: humanTime(post.editedAt()) }));
     if (editedInfo !== this.oldEditedInfo) {
@@ -32,7 +34,17 @@ export default class PostEdited extends Component {
     );
   }
 
-  config(isInitialized) {
+  oncreate(vnode) {
+    super.oncreate(vnode);
+
+    this.rebuildTooltip();
+  }
+
+  onupdate() {
+    this.rebuildTooltip();
+  }
+
+  rebuildTooltip() {
     if (this.shouldUpdateTooltip) {
       this.$().tooltip('destroy').tooltip();
       this.shouldUpdateTooltip = false;