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/ExtensionsPage.js

@@ -12,12 +12,14 @@ export default class ExtensionsPage extends Page {
       <div className="ExtensionsPage">
         <div className="ExtensionsPage-header">
           <div className="container">
-            {Button.component({
-              children: app.translator.trans('core.admin.extensions.add_button'),
-              icon: 'fas fa-plus',
-              className: 'Button Button--primary',
-              onclick: () => app.modal.show(AddExtensionModal),
-            })}
+            {Button.component(
+              {
+                icon: 'fas fa-plus',
+                className: 'Button Button--primary',
+                onclick: () => app.modal.show(AddExtensionModal),
+              },
+              app.translator.trans('core.admin.extensions.add_button')
+            )}
           </div>
         </div>
 
@@ -72,31 +74,35 @@ export default class ExtensionsPage extends Page {
     if (app.extensionSettings[name]) {
       items.add(
         'settings',
-        Button.component({
-          icon: 'fas fa-cog',
-          children: app.translator.trans('core.admin.extensions.settings_button'),
-          onclick: app.extensionSettings[name],
-        })
+        Button.component(
+          {
+            icon: 'fas fa-cog',
+            onclick: app.extensionSettings[name],
+          },
+          app.translator.trans('core.admin.extensions.settings_button')
+        )
       );
     }
 
     if (!enabled) {
       items.add(
         'uninstall',
-        Button.component({
-          icon: 'far fa-trash-alt',
-          children: app.translator.trans('core.admin.extensions.uninstall_button'),
-          onclick: () => {
-            app
-              .request({
-                url: app.forum.attribute('apiUrl') + '/extensions/' + name,
-                method: 'DELETE',
-              })
-              .then(() => window.location.reload());
+        Button.component(
+          {
+            icon: 'far fa-trash-alt',
+            onclick: () => {
+              app
+                .request({
+                  url: app.forum.attribute('apiUrl') + '/extensions/' + name,
+                  method: 'DELETE',
+                })
+                .then(() => window.location.reload());
 
-            app.modal.show(LoadingModal);
+              app.modal.show(LoadingModal);
+            },
           },
-        })
+          app.translator.trans('core.admin.extensions.uninstall_button')
+        )
       );
     }
 
@@ -116,7 +122,7 @@ export default class ExtensionsPage extends Page {
       .request({
         url: app.forum.attribute('apiUrl') + '/extensions/' + id,
         method: 'PATCH',
-        data: { enabled: !enabled },
+        body: { enabled: !enabled },
       })
       .then(() => {
         if (!enabled) localStorage.setItem('enabledExtension', id);