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/admin/components/AdminNav.js

@@ -31,62 +31,74 @@ export default class AdminNav extends Component {
 
     items.add(
       'dashboard',
-      AdminLinkButton.component({
-        href: app.route('dashboard'),
-        icon: 'far fa-chart-bar',
-        children: app.translator.trans('core.admin.nav.dashboard_button'),
-        description: app.translator.trans('core.admin.nav.dashboard_text'),
-      })
+      AdminLinkButton.component(
+        {
+          href: app.route('dashboard'),
+          icon: 'far fa-chart-bar',
+          description: app.translator.trans('core.admin.nav.dashboard_text'),
+        },
+        app.translator.trans('core.admin.nav.dashboard_button')
+      )
     );
 
     items.add(
       'basics',
-      AdminLinkButton.component({
-        href: app.route('basics'),
-        icon: 'fas fa-pencil-alt',
-        children: app.translator.trans('core.admin.nav.basics_button'),
-        description: app.translator.trans('core.admin.nav.basics_text'),
-      })
+      AdminLinkButton.component(
+        {
+          href: app.route('basics'),
+          icon: 'fas fa-pencil-alt',
+          description: app.translator.trans('core.admin.nav.basics_text'),
+        },
+        app.translator.trans('core.admin.nav.basics_button')
+      )
     );
 
     items.add(
       'mail',
-      AdminLinkButton.component({
-        href: app.route('mail'),
-        icon: 'fas fa-envelope',
-        children: app.translator.trans('core.admin.nav.email_button'),
-        description: app.translator.trans('core.admin.nav.email_text'),
-      })
+      AdminLinkButton.component(
+        {
+          href: app.route('mail'),
+          icon: 'fas fa-envelope',
+          description: app.translator.trans('core.admin.nav.email_text'),
+        },
+        app.translator.trans('core.admin.nav.email_button')
+      )
     );
 
     items.add(
       'permissions',
-      AdminLinkButton.component({
-        href: app.route('permissions'),
-        icon: 'fas fa-key',
-        children: app.translator.trans('core.admin.nav.permissions_button'),
-        description: app.translator.trans('core.admin.nav.permissions_text'),
-      })
+      AdminLinkButton.component(
+        {
+          href: app.route('permissions'),
+          icon: 'fas fa-key',
+          description: app.translator.trans('core.admin.nav.permissions_text'),
+        },
+        app.translator.trans('core.admin.nav.permissions_button')
+      )
     );
 
     items.add(
       'appearance',
-      AdminLinkButton.component({
-        href: app.route('appearance'),
-        icon: 'fas fa-paint-brush',
-        children: app.translator.trans('core.admin.nav.appearance_button'),
-        description: app.translator.trans('core.admin.nav.appearance_text'),
-      })
+      AdminLinkButton.component(
+        {
+          href: app.route('appearance'),
+          icon: 'fas fa-paint-brush',
+          description: app.translator.trans('core.admin.nav.appearance_text'),
+        },
+        app.translator.trans('core.admin.nav.appearance_button')
+      )
     );
 
     items.add(
       'extensions',
-      AdminLinkButton.component({
-        href: app.route('extensions'),
-        icon: 'fas fa-puzzle-piece',
-        children: app.translator.trans('core.admin.nav.extensions_button'),
-        description: app.translator.trans('core.admin.nav.extensions_text'),
-      })
+      AdminLinkButton.component(
+        {
+          href: app.route('extensions'),
+          icon: 'fas fa-puzzle-piece',
+          description: app.translator.trans('core.admin.nav.extensions_text'),
+        },
+        app.translator.trans('core.admin.nav.extensions_button')
+      )
     );
 
     return items;