Custom Fields: Allow a field to be marked as read only

Some custom fields should be available as read only fields that cannot be modified by sales reps... but  should be updated / modified via an import or api call.

The reason for this is we may want to present historical data to sales reps but not allow them to accidentally change it or delete it. Also, we use URL links to 3rd party systems that are related to our CRM and we don't want reps adding links to those fields or accidentally editing them. So a read only feature would be very handy.

IDEALLY, this read only would be enforced based on user type permissions... THAT would be the ideal implementation of this suggestions... however, I realize that is more complex, hence why I suggested a simple read only version of this feature for quicker deployment by your devs.