Implement Knocking on Element Web

Links
- PRD (internal): https://docs.google.com/document/d/15ornTrYI5UYVkvlHcD89IIhYIKvdfZcPn6V-KVnMmx8/edit
- Spec: https://spec.matrix.org/v1.2/client-server-api/#knocking-on-rooms
- Figma designs: https://www.figma.com/file/7TqjqdMBaPpm3IavKsMYu6/%5BBWI%5D-Knocking?type=design&node-id=367-50166&mode=design&t=6Bjqu0KINaAjkbV0-0
- Screenshots of proposed design: https://github.com/vector-im/element-web/issues/18655#issuecomment-1095528465 (superceded by Figma??)
- Already implemented by Synapse, just need UI

Mobile (out of scope for this issue): See https://github.com/vector-im/element-android/issues/3875 and https://github.com/vector-im/element-ios/issues/4731
```[tasklist]
### Tasks
- [ ] https://github.com/vector-im/element-web/issues/26043
- [ ] https://github.com/vector-im/element-web/issues/26044
- [ ] https://github.com/vector-im/element-web/issues/26083
- [ ] https://github.com/vector-im/element-web/issues/26084
- [ ] https://github.com/matrix-org/synapse/issues/16307
- [ ] https://github.com/element-hq/element-web/issues/27025
```