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/compat.js

@@ -3,7 +3,6 @@ import compat from '../common/compat';
 import PostControls from './utils/PostControls';
 import KeyboardNavigatable from './utils/KeyboardNavigatable';
 import slidable from './utils/slidable';
-import affixSidebar from './utils/affixSidebar';
 import History from './utils/History';
 import DiscussionControls from './utils/DiscussionControls';
 import alertEmailConfirmation from './utils/alertEmailConfirmation';
@@ -15,6 +14,7 @@ import GlobalSearchState from './states/GlobalSearchState';
 import NotificationListState from './states/NotificationListState';
 import PostStreamState from './states/PostStreamState';
 import SearchState from './states/SearchState';
+import AffixedSidebar from './components/AffixedSidebar';
 import DiscussionPage from './components/DiscussionPage';
 import LogInModal from './components/LogInModal';
 import ComposerBody from './components/ComposerBody';
@@ -61,6 +61,7 @@ import NotificationList from './components/NotificationList';
 import WelcomeHero from './components/WelcomeHero';
 import SignUpModal from './components/SignUpModal';
 import CommentPost from './components/CommentPost';
+import ComposerPostPreview from './components/ComposerPostPreview';
 import ReplyComposer from './components/ReplyComposer';
 import NotificationsPage from './components/NotificationsPage';
 import PostStreamScrubber from './components/PostStreamScrubber';
@@ -77,7 +78,6 @@ export default Object.assign(compat, {
   'utils/PostControls': PostControls,
   'utils/KeyboardNavigatable': KeyboardNavigatable,
   'utils/slidable': slidable,
-  'utils/affixSidebar': affixSidebar,
   'utils/History': History,
   'utils/DiscussionControls': DiscussionControls,
   'utils/alertEmailConfirmation': alertEmailConfirmation,
@@ -89,6 +89,7 @@ export default Object.assign(compat, {
   'states/NotificationListState': NotificationListState,
   'states/PostStreamState': PostStreamState,
   'states/SearchState': SearchState,
+  'components/AffixedSidebar': AffixedSidebar,
   'components/DiscussionPage': DiscussionPage,
   'components/LogInModal': LogInModal,
   'components/ComposerBody': ComposerBody,
@@ -135,6 +136,7 @@ export default Object.assign(compat, {
   'components/WelcomeHero': WelcomeHero,
   'components/SignUpModal': SignUpModal,
   'components/CommentPost': CommentPost,
+  'components/ComposerPostPreview': ComposerPostPreview,
   'components/ReplyComposer': ReplyComposer,
   'components/NotificationsPage': NotificationsPage,
   'components/PostStreamScrubber': PostStreamScrubber,