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/utils/PostControls.js

@@ -61,11 +61,13 @@ export default {
       if (!post.isHidden()) {
         items.add(
           'edit',
-          Button.component({
-            icon: 'fas fa-pencil-alt',
-            children: app.translator.trans('core.forum.post_controls.edit_button'),
-            onclick: this.editAction.bind(post),
-          })
+          Button.component(
+            {
+              icon: 'fas fa-pencil-alt',
+              onclick: this.editAction.bind(post),
+            },
+            app.translator.trans('core.forum.post_controls.edit_button')
+          )
         );
       }
     }
@@ -89,32 +91,38 @@ export default {
       if (post.canHide()) {
         items.add(
           'hide',
-          Button.component({
-            icon: 'far fa-trash-alt',
-            children: app.translator.trans('core.forum.post_controls.delete_button'),
-            onclick: this.hideAction.bind(post),
-          })
+          Button.component(
+            {
+              icon: 'far fa-trash-alt',
+              onclick: this.hideAction.bind(post),
+            },
+            app.translator.trans('core.forum.post_controls.delete_button')
+          )
         );
       }
     } else {
       if (post.contentType() === 'comment' && post.canHide()) {
         items.add(
           'restore',
-          Button.component({
-            icon: 'fas fa-reply',
-            children: app.translator.trans('core.forum.post_controls.restore_button'),
-            onclick: this.restoreAction.bind(post),
-          })
+          Button.component(
+            {
+              icon: 'fas fa-reply',
+              onclick: this.restoreAction.bind(post),
+            },
+            app.translator.trans('core.forum.post_controls.restore_button')
+          )
         );
       }
       if (post.canDelete()) {
         items.add(
           'delete',
-          Button.component({
-            icon: 'fas fa-times',
-            children: app.translator.trans('core.forum.post_controls.delete_forever_button'),
-            onclick: this.deleteAction.bind(post, context),
-          })
+          Button.component(
+            {
+              icon: 'fas fa-times',
+              onclick: this.deleteAction.bind(post, context),
+            },
+            app.translator.trans('core.forum.post_controls.delete_forever_button')
+          )
         );
       }
     }
@@ -128,14 +136,12 @@ export default {
    * @return {Promise}
    */
   editAction() {
-    const deferred = m.deferred();
+    return new Promise((resolve) => {
+      app.composer.load(EditPostComposer, { post: this });
+      app.composer.show();
 
-    app.composer.load(EditPostComposer, { post: this });
-    app.composer.show();
-
-    deferred.resolve(app.composer);
-
-    return deferred.promise;
+      return resolve();
+    });
   },
 
   /**