Improve validation of start & end date for configurations

With the MR !38 (merged) we introduce a validation for start and end dates of the configurations.

However, it doesn't fit on the following scenario:

• I set the start date to 2020-09-25
• I set the end date to 2020-09-24 (one day before)
• => It shows the validation error
• I set the start date to 2020-09-23
• => It still shows the validation error

This should be fixed - and also ensured if you click on the save button.