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/package.json

@@ -3,6 +3,7 @@
   "name": "@flarum/core",
   "dependencies": {
     "@babel/preset-typescript": "^7.10.1",
+    "@types/mithril": "^2.0.3",
     "bootstrap": "^3.4.1",
     "classnames": "^2.2.5",
     "color-thief-browser": "^2.0.2",
@@ -13,7 +14,7 @@
     "jquery.hotkeys": "^0.1.0",
     "lodash-es": "^4.17.14",
     "m.attrs.bidi": "github:tobscure/m.attrs.bidi",
-    "mithril": "^0.2.8",
+    "mithril": "^2.0.4",
     "punycode": "^2.1.1",
     "spin.js": "^3.1.0",
     "webpack": "^4.43.0",