Add date validator

This MR adds an input validator for comparing date inputs with a configured date (absolute or relative).

This makes it possible to only allow date values less/greater than, (not) equal, between (including/excluding threshold values) a given date.

Example scenario: Only allow submitting birth dates of people aged 14-35.

Note: This requires !13 (merged) for correctly building the date objects depending on whether the date input stores the time or not.

merged

mentioned in commit 150a95dd

Thanks