Make all form tasks inherit from Core_Form_Task

[mattwire]

otherwise export can fail when accessed from advanced search because properties are inaccessible.

Overview

Since #11536 export form task uses CRM_Core_Form_Task but because we only converted CRM_Case_Form_Task we can get inaccessible properties when running exports on other entity types.

Identified when investigating #12244

Before

Form_Task classes mostly inherit from CRM_Core_Form.

After

All Form_Task classes inherit from CRM_Core_Form_Task.

Technical Details

This is basically just removal of variables declared on each class and using the parent one instead. We also add comments clarifying that the "plan" is to move more of the code to shared functions in CRM_Core_Form_Task.

[civibot]

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[eileenmcnaughton]

@mattwire I took a look at this & did not manage to cause any problems with or without this patch. I looked & saw these changes were consistent with CRM_Case_Form_Task and CRM_Core_Form_Task_Batch and I'm generally fine with those changes & agree they improve consistency.

I had some reservations about CRM_Export_Form_Select which doesn't feel the same sort of class as the others but I found some precedent in CRM_Event_Form_Task_ParticipantStatus & could not see a specific problem with it