Issue #3004785 by Ishani.addweb, vaccinemedia: Invalid argument supplied error on installation and config form

Added additional permission to allow or disallow editing of the default block class field on block form

Fixed default values for select element and default block class textfield to allow for key|value pairs.
Changed submit function to check if default block class textfield is empty

Issue #1908126 by andrewmacpherson: Select element appears on block form, even if no classes have been defined

Added new select element to handle predefined options while allowing default block_class textfield to still be editabe