Auto-assign reports to office/groups based on custom fields

[philipashlock]

The agency_responsible field should be initialized based on input from the original input.

In the case of the Data.gov help desk we would be getting this from the "Related Agency" custom field used on dataset requests. Currently the Related Agency is enumerated with the agency IDs used on the Fed Agency Directory API (and the POD dashboard) but these can be updated to use the same human readable slugs used for agencies in the help desk.