Refactoring; #267 - prefilled datepicker; disabled possibility to submit an empty date

...locker8/budgetmaster/controller/DatePickerController.java

...eadlocker8/budgetmaster/controller/PaymentController.java

...in/java/de/deadlocker8/budgetmaster/entities/Payment.java

...deadlocker8/budgetmaster/validators/PaymentValidator.java

src/main/resources/languages/_de.properties

src/main/resources/languages/_en.properties

src/main/resources/templates/datePicker.ftl

src/main/resources/templates/payments/newPayment.ftl