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/EventPost.js

@@ -8,28 +8,28 @@ import icon from '../../common/helpers/icon';
  * event, like a discussion being renamed or stickied. Subclasses must implement
  * the `icon` and `description` methods.
  *
- * ### Props
+ * ### Attrs
  *
- * - All of the props for `Post`
+ * - All of the attrs for `Post`
  *
  * @abstract
  */
 export default class EventPost extends Post {
-  attrs() {
-    const attrs = super.attrs();
+  elementAttrs() {
+    const attrs = super.elementAttrs();
 
-    attrs.className = (attrs.className || '') + ' EventPost ' + ucfirst(this.props.post.contentType()) + 'Post';
+    attrs.className = (attrs.className || '') + ' EventPost ' + ucfirst(this.attrs.post.contentType()) + 'Post';
 
     return attrs;
   }
 
   content() {
-    const user = this.props.post.user();
+    const user = this.attrs.post.user();
     const username = usernameHelper(user);
     const data = Object.assign(this.descriptionData(), {
       user,
       username: user ? (
-        <a className="EventPost-user" href={app.route.user(user)} config={m.route}>
+        <a className="EventPost-user" route={app.route.user(user)}>
           {username}
         </a>
       ) : (