Bug 26224: Prevent double submit of header check in form

Bug 26224: Prevent double submit of header check in form

To test:
 1 - Browse to Home
 1 - In the header bar select the 'Check in' tab
 2 - Type a barcode into the box
 3 - Hit Enter as many as times as you can
 4 - Check the statistics table:
    SELECT * FROM statistics WHERE itemnumber={itemnumber} AND DATE(datetime)=CURDATE();
 5 - Note you have multiple lines for the same item at the same time
 6 - Apply patch
 7 - Reload the page
 8 - Type the barcode
 9 - Press Enter even more fast and more furiously
10 - Check the statistics table
11 - Only one entry, huzzah!

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>