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>

framework/core/js/shims.d.ts

@@ -0,0 +1,48 @@
+// Mithril
+import * as Mithril from 'mithril';
+import Stream from 'mithril/stream';
+
+// Other third-party libs
+import * as _dayjs from 'dayjs';
+import * as _$ from 'jquery';
+
+// Globals from flarum/core
+import Application from './src/common/Application';
+
+/**
+ * Helpers that flarum/core patches into Mithril
+ */
+interface m extends Mithril.Static {
+  prop: typeof Stream;
+}
+
+/**
+ * Export Mithril typings globally.
+ *
+ * This lets us use these typings without an extra import everywhere we use
+ * Mithril in a TypeScript file.
+ */
+export as namespace Mithril;
+
+/**
+ * flarum/core exposes several extensions globally:
+ *
+ * - jQuery for convenient DOM manipulation
+ * - Mithril for VDOM and components
+ * - dayjs for date/time operations
+ *
+ * Since these are already part of the global namespace, extensions won't need
+ * to (and should not) bundle these themselves.
+ */
+declare global {
+  const $: typeof _$;
+  const m: m;
+  const dayjs: typeof _dayjs;
+}
+
+/**
+ * All global variables owned by flarum/core.
+ */
+declare global {
+  const app: Application;
+}