WebUI: Show firmware update notification in WebUI

Displays update notification for user, but does not provide
automatic updates. Version comparison is based on string match,
so build dates are not relevant.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

Tested-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Michael Fritscher <fff@mifritscher.de>