Keyboard shortcut for "Mark all as read"

[davidtheclark]

I would love a keyboard shortcut equivalent to the "Mark all as read" checkmark button. Are you open to this idea? I could try a PR if so.

"a" seems logical but I wonder if this action should require a little extra work, as you don't want to hit it by accident. Following the g p, g e, etc. precedent, how about m a?