Move the code to add or clear employer from relationship backoffice form to BAO

Move the code to add or clear employer from relationship backoffice form to BAO

This is a reduced version of https://github.com/civicrm/civicrm-core/pull/13331 which stalled on the QA over handling of
removed employers - that code is simply excluded. I tested adding & removing & editing relationships with the
current_employer checkbox ticked or unticked & this seems to work fine