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/ConfirmDocumentUnload.js

@@ -5,7 +5,7 @@ import Component from '../Component';
  * event handler that prevents closing the browser window/tab based on the
  * return value of a given callback prop.
  *
- * ### Props
+ * ### Attrs
  *
  * - `when` - a callback returning true when the browser should prompt for
  *            confirmation before closing the window/tab
@@ -17,21 +17,24 @@ import Component from '../Component';
  *
  */
 export default class ConfirmDocumentUnload extends Component {
-  config(isInitialized, context) {
-    if (isInitialized) return;
-
-    const handler = () => this.props.when() || undefined;
-
-    $(window).on('beforeunload', handler);
-
-    context.onunload = () => {
-      $(window).off('beforeunload', handler);
-    };
+  handler() {
+    return this.attrs.when() || undefined;
   }
 
-  view() {
+  oncreate(vnode) {
+    super.oncreate(vnode);
+
+    this.boundHandler = this.handler.bind(this);
+    $(window).on('beforeunload', this.boundHandler);
+  }
+
+  onremove() {
+    $(window).off('beforeunload', this.boundHandler);
+  }
+
+  view(vnode) {
     // To avoid having to render another wrapping <div> here, we assume that
     // this component is only wrapped around a single element / component.
-    return this.props.children[0];
+    return vnode.children[0];
   }
 }