Add pattern to US zip code input, to trigger numeric keypad on iOS

Add pattern="\d*" to the input element for US zip codes.
This will cause iOS to show a numeric keyboard
(unfortunately doesn't seem to have any effect on Android)

Not added for any non-US postal codes as they all seem to contain
letters and/or hyphens

Change-Id: I593202c7db221eb86e8c7f2da0aa1c144cfa0f3d