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/states/PostStreamState.js

@@ -47,7 +47,7 @@ class PostStreamState {
    * @public
    */
   update() {
-    if (!this.viewingEnd()) return m.deferred().resolve().promise;
+    if (!this.viewingEnd()) return Promise.resolve();
 
     this.visibleEnd = this.count();
 
@@ -134,7 +134,7 @@ class PostStreamState {
    */
   loadNearNumber(number) {
     if (this.posts().some((post) => post && Number(post.number()) === Number(number))) {
-      return m.deferred().resolve().promise;
+      return Promise.resolve();
     }
 
     this.reset();
@@ -157,7 +157,7 @@ class PostStreamState {
    */
   loadNearIndex(index) {
     if (index >= this.visibleStart && index <= this.visibleEnd) {
-      return m.deferred().resolve().promise;
+      return Promise.resolve();
     }
 
     const start = this.sanitizeIndex(index - this.constructor.loadCount / 2);
@@ -229,7 +229,7 @@ class PostStreamState {
         this.loadRange(start, end).then(() => {
           if (start >= this.visibleStart && end <= this.visibleEnd) {
             const anchorIndex = backwards ? this.visibleEnd - 1 : this.visibleStart;
-            anchorScroll(`.PostStream-item[data-index="${anchorIndex}"]`, () => m.redraw(true));
+            anchorScroll(`.PostStream-item[data-index="${anchorIndex}"]`, () => m.redraw.sync());
           }
           this.pagesLoading--;
         });
@@ -266,7 +266,7 @@ class PostStreamState {
         }
       });
 
-    return loadIds.length ? app.store.find('posts', loadIds) : m.deferred().resolve(loaded).promise;
+    return loadIds.length ? app.store.find('posts', loadIds) : Promise.resolve(loaded);
   }
 
   /**