Bug 25349: Add checkEnter function and apply it to SCO page

Bug 25349: Add checkEnter function and apply it to SCO page

This patch adds the checkEnter function to the OPAC global.js and then applies it to the SCO login page.
TO TEST:
1. Go to SCO login and try entering something in the "Login:" input amd "Password:" input.
2. Pressing enter while in either will automatically submit the form.
3. Apply patch.
4. Type something into the Login: field and press enter, the form should NOT submit.
5. Type something into the Password: field and pess enter, this SHOULD submit the form.
6. Change the syspref SelfCheckoutByLogin to 'Cardnumber'
7. Type something into the Please enter your card number: field, the form should submit.

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