Add paid flag to email subject, check receipt date in tests

config/locales/en.yml

@@ -470,7 +470,8 @@ en:
   price: 'Price'
   total: 'Total'
   paid_at: 'Paid at'
-  invoice: 'Invoice'
+  invoice:
+    already_paid: " (already paid)"
   bank_statements: 'Bank statements'
   back_to_bank_statements: 'Back to bank statements'
   back_to_bank_statement: 'Back to bank statement'

config/locales/et.yml

@@ -6,3 +6,5 @@ et:
     # Don't forget the nil at the beginning; there's no such thing as a 0th month
     month_names: [~, Jaanuar, Veebruar, Märts, Aprill, Mai, Juuni, Juuli, August, September, Oktoober, November, Detsember]
   emails: "Meillaadressid"
+  invoice:
+    already_paid: " (juba makstud)"