Change parent class on mailing_form

[eileenmcnaughton]

Overview

Just changes the parent class to be more like the other search forms. Also adds the call to getSearchFieldMetadata but at this stage it's not using it to add fields

Before

Extends CRM_Core_Form

After

Extends CRM_Core_Form_Search

Technical Details

From https://github.com/civicrm/civicrm-core/pull/15369/files

Comments

I did some tests & it seemed to change nothing = good

[civibot]

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[seamuslee001]

(CiviCRM Review Template WORD-1.2)

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