ext.echo.overlay: Only build the 'mark all as read' button if it's used

Otherwise a new copy will be created every time the user opens and closes
the popup, and jQuery will hold on to each one forever (because it has
events bound), leaking a minuscule amount of memory.

Change-Id: I70c713be839f826fc27d07b04260c166f9052020
Merge "ext.echo.overlay: Only build the 'mark all as read' button if it's used"