Bug 30619: Add email receipt to POS

Bug 30619: Add email receipt to POS

This patch adds the ability to use email receipts with the point of sale
module.

To test:
1) Configure your Koha to enable the point of sale system.
2) Add an email template to the 'Point of sale > RECEIPT' notice.
3) Add some charges on point of sale and pay for them.
3a) You should now see a new 'Email receipt' button next to the 'Print
receipt' option.
3b) Clicking the button will display a modal to enter the anonymous
users email address
3c) Enter an email address and confirm to send the email

Sponsored-by: Martin Renvoize <martin.renvoize@gmail.com>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>