Update enrollment code confirmation emails

lms/templates/emails/registration_codes_sale_email.txt

 <%! from django.utils.translation import ugettext as _ %>
 
-${_("Thank you for your purchase of {course_name}!").format(course_name=course.display_name)}
+${_("Thank you for purchasing enrollments in {course_name}.").format(course_name=course.display_name)}
 
-${_("An invoice for {currency_symbol}{total_price} is attached. Payment is due immediately.  Information on payment methods can be found on the invoice.").format(currency_symbol=currency_symbol, total_price=sale_price)}
+${_("An invoice for {currency_symbol}{total_price} is attached. Payment is due upon receipt. You can find information about payment methods on the invoice.").format(currency_symbol=currency_symbol, total_price=sale_price)}
 
-${_("A CSV file of your registration codes is attached. Please distribute registration codes to each student planning to enroll using the email template below.")}
+${_("A .csv file that lists your enrollment codes is attached. You can use the email template below to distribute enrollment codes to your students. Each student must use a separate enrollment code.")}
 
 ## Translators: This is the signature of an email.  "\n" is a newline character
 ## and should be placed between the closing word and the signature.

lms/templates/emails/registration_codes_sale_invoice_attachment.txt

@@ -7,7 +7,7 @@ ${corp_address}
 
 ${_("Date: {date}").format(date=date)}
 ${_("Invoice No: {invoice_number}").format(invoice_number=invoice.id)}
-${_("Terms: Due Immediately")}
+${_("Terms: Due Upon Receipt")}
 ${_("Due Date: {date}").format(date=date)}
 
 ${_("Bill to:")}
@@ -33,10 +33,10 @@ ${_("Total:      {currency_symbol}{sale_price}").format(sale_price=sale_price, c
 
 ———————————————————————————————————————————
 
-${_("Payment instructions")}
+${_("Payment Instructions")}
 
 ${payment_instructions}
 
-${_("Failure to pay this invoice will result the invalidation of student enrollment that use these codes. All purchases are final. Please refer to the cancellation policy on {site_name} for more information.").format(site_name=site_name)}
+${_("If we do not receive payment, the learner enrollments that use these codes will be canceled and learners will not be able to access course materials. All purchases are final. For more information, see the {site_name} cancellation policy.").format(site_name=site_name)}
 
-${_("If you have payment questions, please contact {contact_email}").format(contact_email=contact_email)}
+${_("For payment questions, contact {contact_email}").format(contact_email=contact_email)}