AWAY should be per-session

[slingamn]

@kylef points out that the existing mechanism for auto-communicating presence to the server is the AWAY command, and many clients already have auto-away capabilities.

Particularly in the context of #1527 (but maybe in other contexts as well) we need a way to suppress push notifications when the user is actually present. Here's the suggested design:

1. Each session will independently have an away status and an away message
2. You're not away unless all your sessions are away (or you have no sessions and the auto-away preference is set)
3. If you become away, we publicly display the away message of the last session to become away