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
Make cart save redirect to cart summary

Currently the cart save action doesn't follow the POST/redirect pattern.
Remove hard dependency on mongodb
Merge pull request #383 from stloyd/bugfix/mongo_dep

Remove hard dependency on mongodb
New backend theme with Bootstrap 3
Update translations
Update translations, start reworking the product page
Some fixes and adjustments
Add initial breadcrumbs
Additional polishing, some hardcoded styles removal.
More breadcrumbs
Initial translations
Add missing breadcrumbs
Fix missing translations
Fix some broken scenarios