Add support for MD / HTML in room topics (#8215)

* Add support for MD / HTML in room topics

Setting MD / HTML supported:
- /topic command
- Room settings overlay
- Space settings overlay

Display of MD / HTML supported:
- /topic command
- Room header
- Space home

Based on extensible events as defined in [MSC1767]

Fixes: vector-im/element-web#5180
Signed-off-by: Johannes Marbach <johannesm@element.io>

[MSC1767]: matrix-org/matrix-spec-proposals#1767

* Fix build error

* Add comment to explain origin of styles

Co-authored-by: Travis Ralston <travpc@gmail.com>

* Empty commit to retrigger build

* Fix import grouping

* Fix useTopic test

* Add tests for HtmlUtils

* Add slash command test

* Add further serialize test

* Fix ternary formatting

Co-authored-by: Travis Ralston <travpc@gmail.com>

* Add blank line

Co-authored-by: Travis Ralston <travpc@gmail.com>

* Properly mock SettingsStore access

* Remove trailing space

* Assert on HTML content and add test for plain text in HTML parameter

* Appease the linter

* Fix JSDoc comment

* Fix toEqual call formatting

* Repurpose test for literal HTML case

* Empty commit to fix CI

Co-authored-by: Travis Ralston <travpc@gmail.com>
Co-authored-by: Travis Ralston <travisr@matrix.org>

src/settings/Settings.tsx

@@ -330,6 +330,13 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         supportedLevels: [SettingLevel.ACCOUNT],
         default: null,
     },
+    "feature_html_topic": {
+        isFeature: true,
+        labsGroup: LabGroup.Rooms,
+        supportedLevels: LEVELS_FEATURE,
+        displayName: _td("Show HTML representation of room topics"),
+        default: false,
+    },
     "feature_bridge_state": {
         isFeature: true,
         labsGroup: LabGroup.Rooms,