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/editor/serialize-test.ts

@@ -94,5 +94,12 @@ describe('editor/serialize', function() {
             const html = htmlSerializeIfNeeded(model, { useMarkdown: false });
             expect(html).toBe('\\*hello\\* world < hey world!');
         });
+
+        it('plaintext remains plaintext even when forcing html', function() {
+            const pc = createPartCreator();
+            const model = new EditorModel([pc.plain("hello world")], pc);
+            const html = htmlSerializeIfNeeded(model, { forceHTML: true, useMarkdown: false });
+            expect(html).toBe("hello world");
+        });
     });
 });