Improvement of presence markers UI

[Ezio-Sarthak]

After the successful merge of #987, we support a simple UI for presence markers. Here are some thoughts for some follow-ups that'd need discussion and implement promptly:

• Show bot markers (this was also suggested in role-markers PR)
• Handle "unavailable" visibility and explicit changes (are they events? See Handle user update events & consequential UI changes #988)
• Only update the status rather than the entire box (maybe not that tricky?)
• Show user presence in Direct message compose?
• Show user presence in user mentions?
• Show user presence in Direct message narrow heading/title for each user

Feel free to hop-in for any discussion on CZO

Tagging with GSoC, as elements might be a good project during summer.

[neiljp]

Note that the PM compose and user mention cases also could include autocomplete, which is what the web app currently does - it doesn't include user status in rendered mentions.