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>

test/test-utils/test-utils.ts

@@ -116,6 +116,7 @@ export function createTestClient(): MatrixClient {
         mxcUrlToHttp: (mxc) => `http://this.is.a.url/${mxc.substring(6)}`,
         setAccountData: jest.fn(),
         setRoomAccountData: jest.fn(),
+        setRoomTopic: jest.fn(),
         sendTyping: jest.fn().mockResolvedValue({}),
         sendMessage: () => jest.fn().mockResolvedValue({}),
         sendStateEvent: jest.fn().mockResolvedValue(undefined),