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/PostsUserPage.js

@@ -9,8 +9,8 @@ import CommentPost from './CommentPost';
  * profile.
  */
 export default class PostsUserPage extends UserPage {
-  init() {
-    super.init();
+  oninit(vnode) {
+    super.oninit(vnode);
 
     /**
      * Whether or not the activity feed is currently loading.
@@ -55,15 +55,13 @@ export default class PostsUserPage extends UserPage {
     let footer;
 
     if (this.loading) {
-      footer = LoadingIndicator.component();
+      footer = <LoadingIndicator />;
     } else if (this.moreResults) {
       footer = (
         <div className="PostsUserPage-loadMore">
-          {Button.component({
-            children: app.translator.trans('core.forum.user.posts_load_more_button'),
-            className: 'Button',
-            onclick: this.loadMore.bind(this),
-          })}
+          <Button className="Button" onclick={this.loadMore.bind(this)}>
+            {app.translator.trans('core.forum.user.posts_load_more_button')}
+          </Button>
         </div>
       );
     }
@@ -75,14 +73,11 @@ export default class PostsUserPage extends UserPage {
             <li>
               <div className="PostsUserPage-discussion">
                 {app.translator.trans('core.forum.user.in_discussion_text', {
-                  discussion: (
-                    <a href={app.route.post(post)} config={m.route}>
-                      {post.discussion().title()}
-                    </a>
-                  ),
+                  discussion: <a route={app.route.post(post)}>{post.discussion().title()}</a>,
                 })}
               </div>
-              {CommentPost.component({ post })}
+
+              <CommentPost post={post} />
             </li>
           ))}
         </ul>
@@ -110,7 +105,7 @@ export default class PostsUserPage extends UserPage {
     this.loading = true;
     this.posts = [];
 
-    m.lazyRedraw();
+    m.redraw();
 
     this.loadResults().then(this.parseResults.bind(this));
   }