Afform: Only permit Date fields to be made into a select list if it i…

[seamuslee001]

…s a search form

Overview

This limits the forms that can have date fields as a select list to just search forms

Before

Any Afform can have a date field as a select list

After

Only search display afforms can have date field as a select list

ping @eileenmcnaughton @colemanw @JoeMurray

[civibot]

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[colemanw]

Test failure unrelated.

This is clever @seamuslee001. That function may or may not need tweaking someday as our metadata models evolve, but since you've encapsulated the logic in a function, that makes it easy to update in the future.

For now, it works and it's an improvement so I'm merging it.