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/ComposerPostPreview.js

@@ -0,0 +1,54 @@
+/*global s9e*/
+
+import Component from '../../common/Component';
+
+/**
+ * The `ComposerPostPreview` component renders Markdown as HTML using the
+ * TextFormatter library, polling a data source for changes every 50ms. This is
+ * done to prevent expensive redraws on e.g. every single keystroke, while
+ * still retaining the perception of live updates for the user.
+ *
+ * ### Attrs
+ *
+ * - `composer` The state of the composer controlling this preview.
+ * - `className` A CSS class for the element surrounding the preview.
+ * - `surround` A callback that can execute code before and after re-render, e.g. for scroll anchoring.
+ */
+export default class ComposerPostPreview extends Component {
+  static initAttrs(attrs) {
+    attrs.className = attrs.className || '';
+    attrs.surround = attrs.surround || ((preview) => preview());
+  }
+
+  view() {
+    return <div className={this.attrs.className} />;
+  }
+
+  oncreate(vnode) {
+    super.oncreate(vnode);
+
+    // Every 50ms, if the composer content has changed, then update the post's
+    // body with a preview.
+    let preview;
+    const updatePreview = () => {
+      // Since we're polling, the composer may have been closed in the meantime,
+      // so we bail in that case.
+      if (!this.attrs.composer.isVisible()) return;
+
+      const content = this.attrs.composer.fields.content();
+
+      if (preview === content) return;
+
+      preview = content;
+
+      this.attrs.surround(() => s9e.TextFormatter.preview(preview || '', vnode.dom));
+    };
+    updatePreview();
+
+    this.updateInterval = setInterval(updatePreview, 50);
+  }
+
+  onremove() {
+    clearInterval(this.updateInterval);
+  }
+}