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/Search.js

@@ -16,13 +16,14 @@ import UsersSearchSource from './UsersSearchSource';
  * getInitialSearch() value is a truthy value. If this is the case, an 'x'
  * button will be shown next to the search field, and clicking it will clear the search.
  *
- * PROPS:
+ * ATTRS:
  *
  * - state: SearchState instance.
  */
 export default class Search extends Component {
-  init() {
-    this.state = this.props.state;
+  oninit(vnode) {
+    super.oninit(vnode);
+    this.state = this.attrs.state;
 
     /**
      * Whether or not the search input has focus.
@@ -86,7 +87,7 @@ export default class Search extends Component {
             type="search"
             placeholder={extractText(app.translator.trans('core.forum.header.search_placeholder'))}
             value={this.state.getValue()}
-            oninput={m.withAttr('value', this.state.setValue.bind(this.state))}
+            oninput={(e) => this.state.setValue(e.target.value)}
             onfocus={() => (this.hasFocus = true)}
             onblur={() => (this.hasFocus = false)}
           />
@@ -107,15 +108,20 @@ export default class Search extends Component {
     );
   }
 
-  config(isInitialized) {
+  onupdate() {
     // Highlight the item that is currently selected.
     this.setIndex(this.getCurrentNumericIndex());
+  }
 
-    if (isInitialized) return;
+  oncreate(vnode) {
+    super.oncreate(vnode);
 
     const search = this;
     const state = this.state;
 
+    // Highlight the item that is currently selected.
+    this.setIndex(this.getCurrentNumericIndex());
+
     this.$('.Search-results')
       .on('mousedown', (e) => e.preventDefault())
       .on('click', () => this.$('input').blur())
@@ -179,7 +185,7 @@ export default class Search extends Component {
     this.loadingSources = 0;
 
     if (this.state.getValue()) {
-      m.route(this.getItem(this.index).find('a').attr('href'));
+      m.route.set(this.getItem(this.index).find('a').attr('href'));
     } else {
       this.clear();
     }