Add missing gender field in new customer form

[holybiber]

Customers -> New customer was missing the gender field. Filling this form out resulted in a QueryException: Integrity constraint violation: 19 NOT NULL constraint failed: shop_customers.gender Fixing this by adding the gender field to this form. This is similar to PR #64 which fixed a similar issue when adding a customer while creating a new order (Orders -> New order -> Adding a customer there)