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/i18n/strings/en_EN.json

@@ -892,6 +892,7 @@
     "Offline encrypted messaging using dehydrated devices": "Offline encrypted messaging using dehydrated devices",
     "Show extensible event representation of events": "Show extensible event representation of events",
     "Show current avatar and name for users in message history": "Show current avatar and name for users in message history",
+    "Show HTML representation of room topics": "Show HTML representation of room topics",
     "Show info about bridges in room settings": "Show info about bridges in room settings",
     "Use new room breadcrumbs": "Use new room breadcrumbs",
     "New search experience": "New search experience",