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.

首先，在 GitHub App 中复现 session 的已读/未读行为，包括合并的 session、多选以及应用图标的未读指示器。明确预期采用自动标记、多选更新和“全部标记为已读”操作中的哪种组合；当约定的行为能够一致地工作，并且指示器反映出这一点时，即表示完成。

由索引模型根据 Issue 内容生成。