Make it easier/configuratble to mark sessions as read

I like that sessions have a "read/unread" state now, but now dismissing becomes a thing. For the merged sessions I really do not need a state at all, so that could be marked as read for me automatically.

CTRL-selecting multiple sessions could be an option, but doing so and selecting read/unread does only toggle the state for the session I selected last.

A "mark all as read" option would already help.

## What got me here?
Being annoyed by the "9+" indicator on the App icon, which I though was referring to the unread sessions. I closed the ones that I could see, but the 9+ indicator is still there. Scrolling down I found more sessions to mark as read.

Start by reproducing the session read/unread behavior in the GitHub App, including merged sessions, multi-selection, and the app icon's unread indicator. Clarify which combination of automatic marking, multi-select updates, and a “mark all as read” action is intended; done means the agreed behavior works consistently and the indicator reflects it.

索引モデルが issue の本文から書いたものです。