Date: Allow setting relative dates for earliest and latest through field editor

[FrittenKeeZ]

Since earliest_date and latest_date are already parsed through Carbon, allowing for relative values like now, tomorrow, +2 months etc., it would be nice to be able to specify Allow relative format (true/false) as date configuration in field editor, which adds a button next to "Add Date" saying "Add Relative". This would then open a simple text input where you can write the relative date format when filling out the date.