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.

Empieza reproduciendo el comportamiento de lectura/no lectura de las sesiones en la GitHub App, incluidas las sesiones combinadas, la selección múltiple y el indicador de no leídos del icono de la app. Aclara qué combinación de marcado automático, actualizaciones mediante selección múltiple y una acción de “marcar todo como leído” está prevista; se considera completado cuando el comportamiento acordado funciona de forma coherente y el indicador lo refleja.

Escrito por el modelo de indexación a partir del texto del issue.