Resolve "Improve validation of start & end date for configurations"

Closes #23 (closed)

Goals of this MR:

• fixes the date validation as described in #23 (closed)
• shows an error and rejects saving when the form is not valid

Sidenote: The way the validation is implemented now, both date fields get marked in red, which is technically correct but somewhat unexpected (before this change, only the field in which the invalid value was entered got marked in red). But I hope that's okay...