Remove "mark all as read" confirmation, but allow the action to be undone

I get really frustrated by the "mark all as read" confirmation - it turns a common task from one click into two.

Let's remove the confirmation, but show an alert when clicked ("All discussions were marked as read") with an undo button which will revert the change. We should also show a tooltip on hover of the button, to make it more obvious what the button does.

Start by locating the “mark all as read” control and its existing confirmation flow. Implement the requested alert with an undo action and a hover tooltip, then verify that marking all discussions read and reverting the action behave as described.

Written by the indexing model from the issue text.