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/PostStream.js

@@ -8,7 +8,7 @@ import Button from '../../common/components/Button';
  * The `PostStream` component displays an infinitely-scrollable wall of posts in
  * a discussion. Posts that have not loaded will be displayed as placeholders.
  *
- * ### Props
+ * ### Attrs
  *
  * - `discussion`
  * - `stream`
@@ -16,9 +16,11 @@ import Button from '../../common/components/Button';
  * - `onPositionChange`
  */
 export default class PostStream extends Component {
-  init() {
-    this.discussion = this.props.discussion;
-    this.stream = this.props.stream;
+  oninit(vnode) {
+    super.oninit(vnode);
+
+    this.discussion = this.attrs.discussion;
+    this.stream = this.attrs.stream;
 
     this.scrollListener = new ScrollListener(this.onscroll.bind(this));
   }
@@ -96,29 +98,33 @@ export default class PostStream extends Component {
     return <div className="PostStream">{items}</div>;
   }
 
-  config(isInitialized, context) {
+  onupdate() {
     this.triggerScroll();
+  }
 
-    if (isInitialized) return;
+  oncreate(vnode) {
+    super.oncreate(vnode);
+
+    this.triggerScroll();
 
     // This is wrapped in setTimeout due to the following Mithril issue:
     // https://github.com/lhorie/mithril.js/issues/637
     setTimeout(() => this.scrollListener.start());
+  }
 
-    context.onunload = () => {
-      this.scrollListener.stop();
-      clearTimeout(this.calculatePositionTimeout);
-    };
+  onremove() {
+    this.scrollListener.stop();
+    clearTimeout(this.calculatePositionTimeout);
   }
 
   /**
    * Start scrolling, if appropriate, to a newly-targeted post.
    */
   triggerScroll() {
-    if (!this.props.targetPost) return;
+    if (!this.attrs.targetPost) return;
 
     const oldTarget = this.prevTarget;
-    const newTarget = this.props.targetPost;
+    const newTarget = this.attrs.targetPost;
 
     if (oldTarget) {
       if ('number' in oldTarget && oldTarget.number === newTarget.number) return;
@@ -265,7 +271,7 @@ export default class PostStream extends Component {
     });
 
     if (startNumber) {
-      this.props.onPositionChange(startNumber || 1, endNumber, startNumber);
+      this.attrs.onPositionChange(startNumber || 1, endNumber, startNumber);
     }
   }
 
@@ -348,7 +354,7 @@ export default class PostStream extends Component {
     return Promise.all([$container.promise(), this.stream.loadPromise]).then(() => {
       this.updateScrubber();
       const index = $item.data('index');
-      m.redraw(true);
+      m.redraw.sync();
       const scroll = index == 0 ? 0 : $(`.PostStream-item[data-index=${$item.data('index')}]`).offset().top - this.getMarginTop();
       $(window).scrollTop(scroll);
       this.calculatePosition();