This commit implements a label and a text box for room topic that can be used to display and edit it.

Type: add

BUG: https://iron.lily-is.land/T109

Verify unit tests pass. In the room settings page:

• In a room where there is no room topic, you should see a label saying the room does not have a topic.
• In a room where there is a topic, you should see the topic.
• In a room where you can change room topic, set the topic and verify it is updated.
• In a room where you cannot change room topic, set the topic and verify it reports an error.