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/ChangeEmailModal.js

@@ -6,8 +6,8 @@ import Button from '../../common/components/Button';
  * to change their email address.
  */
 export default class ChangeEmailModal extends Modal {
-  init() {
-    super.init();
+  oninit(vnode) {
+    super.oninit(vnode);
 
     /**
      * Whether or not the email has been changed successfully.
@@ -21,14 +21,14 @@ export default class ChangeEmailModal extends Modal {
      *
      * @type {function}
      */
-    this.email = m.prop(app.session.user.email());
+    this.email = m.stream(app.session.user.email());
 
     /**
      * The value of the password input.
      *
      * @type {function}
      */
-    this.password = m.prop('');
+    this.password = m.stream('');
   }
 
   className() {
@@ -81,12 +81,14 @@ export default class ChangeEmailModal extends Modal {
             />
           </div>
           <div className="Form-group">
-            {Button.component({
-              className: 'Button Button--primary Button--block',
-              type: 'submit',
-              loading: this.loading,
-              children: app.translator.trans('core.forum.change_email.submit_button'),
-            })}
+            {Button.component(
+              {
+                className: 'Button Button--primary Button--block',
+                type: 'submit',
+                loading: this.loading,
+              },
+              app.translator.trans('core.forum.change_email.submit_button')
+            )}
           </div>
         </div>
       </div>
@@ -122,7 +124,7 @@ export default class ChangeEmailModal extends Modal {
 
   onerror(error) {
     if (error.status === 401) {
-      error.alert.children = app.translator.trans('core.forum.change_email.incorrect_password_message');
+      error.alert.content = app.translator.trans('core.forum.change_email.incorrect_password_message');
     }
 
     super.onerror(error);