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/common/components/Modal.js

@@ -14,23 +14,21 @@ export default class Modal extends Component {
    */
   static isDismissible = true;
 
-  init() {
-    /**
-     * Attributes for an alert component to show below the header.
-     *
-     * @type {object}
-     */
-    this.alertAttrs = null;
+  /**
+   * Attributes for an alert component to show below the header.
+   *
+   * @type {object}
+   */
+  alertAttrs = null;
+
+  oncreate(vnode) {
+    super.oncreate(vnode);
+
+    this.attrs.onshow(() => this.onready());
   }
 
-  config(isInitialized, context) {
-    if (isInitialized) return;
-
-    this.props.onshow(() => this.onready());
-
-    context.onunload = () => {
-      this.props.onhide();
-    };
+  onremove() {
+    this.attrs.onhide();
   }
 
   view() {
@@ -109,7 +107,7 @@ export default class Modal extends Component {
    * Hide the modal.
    */
   hide() {
-    this.props.onhide();
+    this.attrs.onhide();
   }
 
   /**