Add backend form error messages

Currently global form errors (extra fields, csrf, etc) are not displayed in backend.
This commit adds one to discuss if this is the desired approach. (will do all if OK).
Add global form errors to backend forms
Adding global form errors to some Frontend forms
Fix CreditCardType to work with Symfony's Luhn Validator

Currently the number field is of type number, but Symfony's Luhn validator only works with strings.
Consequently this field fails to validate. This commit changes the type to text.
Merge pull request #371 from Richtermeister/fix-credit-card-form

Fix CreditCardType to work with Symfony's Luhn Validator
Merge pull request #346 from Richtermeister/backend-form-errors

Add backend form error messages
Remove hard dependency on mongodb