Success Plan: Make custom fields mappable in rule creation but not editable on front end

The current setup, if you want to insert information to a custom field on a success plan, it needs to be editable from the UI on the plan itself.

There are some data that we want to import that we do not want editable from the UI, but we want to update with a rule.

For example, we want to update a custom field that indicates what quarter the the plan is generated for via a rule, but do not want CSM to alter this once the plan is generated.