Add api testing / custom data support for MailingJob.

Add api testing / custom data support for MailingJob.

There is no specific requirement for MailingJob to have custom data - but
this is generally part of standardising all entities to support custom
data (ie. workign through the list in
api_v3_SyntaxConformanceTest:toBeSkipped_custom_data_creatable
 to remove the remaining rows.

Fix some places to call create instead of saving mailing job to trigger hooks.