Add unique constraint on UnitRespondingAgency

[hancush]

This PR leverages unique_together to add a unique constraint on UnitRespondingAgency, such that units may only be associated with one responding agency per year.

                                   Table "public.payroll_unitrespondingagency"
        Column        |  Type   | Collation | Nullable |                         Default
----------------------+---------+-----------+----------+----------------------------------------------------------
 id                   | integer |           | not null | nextval('payroll_unitrespondingagency_id_seq'::regclass)
 reporting_year       | integer |           | not null |
 responding_agency_id | integer |           | not null |
 unit_id              | integer |           | not null |
Indexes:
...
    "payroll_unitrespondingag_unit_id_reporting_year_988eebd6_uniq" UNIQUE CONSTRAINT, btree (unit_id, reporting_year)
...

Related to #324.