[#1633] Add filtering on legacy PartnershipResource.partner_type field
[#1633] Add filtering on legacy PartnershipResource.partner_type field

Enable filtering on legacy Partnership.partner_type(_2) values in the
Tastypie PartnershipResource

NOTE: when the Partnership.partner_type field is removed, all
references to partner_type_2 should be changed to partner_type

akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr