Use avatars in DMs navbar

Instead of names (which are visible in the message header bar), we should try using avatars in the navbar for DM conversations.

• We should show an availability dot on the bottom right of the avatars, as we do in the right sidebar (and elsewhere).
• Clicking on an avatar should open the user card.

We should show both the avatar and the name in 1:1 DMs.

Notes

• Keyboard selection outlines should have rounded corners, like elsewhere in the app, as should the names listed in the tooltip we show when not all the avatars fit.
• Avatars should have the same order as names in the message header bar.
• Presence circle should be the same size as in the right sidebar (show avatar mode).
• We should leave some breathing room between avatars when there's more than one, as we generally don't expect horizontal space to be a problem. See #design > availability and status emoji in DM views @ 💬.

Questions

• Do we want to keep the DMs icon in the navbar or drop it? We should probably look at both options.
• Should we show the name as well for 1:1 DMs?

CZO thread: #design > availability and status emoji in DM views