MDL-62414 tool_dataprivacy: Only show user's children in mydatarequests

MDL-62414 tool_dataprivacy: Only show user's children in mydatarequests

* When coming from the DPO's "My data requests" page, don't show the
  picker for other users (unless the DPO is a parent of another user).
* When creating a data request as a DPO, show a more appropriate
  notification after redirecting back to the data requests page.