Assign topic (URL) to a chat

When starting a chat (or at any later point), allow the user to optionally specify a topic (URL). This would make chats discoverable by topic and help relate different conversations.

Use properties like schema:about, foaf:topic, dcterms:subject, as:inReplyTo, sioc:reply_of, or similar.

While playing with dokieli's notifications feature - which surfaces activities and interactions from both my contacts and myself with the current document - I realised this could also be used to discover chats related to a page and display them in the notifications side panel. This is conceptually similar to how Activity Streams uses object or target, depending on the activity type.

For example, if I can locate Tim's LongChat via his TypeIndex (assuming I have access), and identify the one tied to a specific topic, I could show that thread in the side panel. The user could then read or post new messages from there.

Start by reviewing how chats are created and located, then read dokieli's notifications feature and the TypeIndex discussion referenced in the issue. Compare the proposed topic properties and Activity Streams relationships before deciding how a topic should be stored and discovered. Done means a chat can receive an optional topic URL and related conversations can appear in the notifications side panel.