[Ref] [Import] Simplify form event MapField to use shared rule (Participant)

[eileenmcnaughton]

Overview

[Ref] [Import] Simplify form event MapField to use shared rule

Before

Copy and paste

After

Validation on the savedMapping uses the shared validation function (common to most imports as this change has been made to most already)

Technical Details

the other outstanding one is contact - I'll put up a PR for that

Comments

[civibot]

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[eileenmcnaughton]

@colemanw similar to others you have merged