Update custom attribute handler
Update custom attribute handler

Make custom attribute imports work for new object and for simple custom
attributes. Dropdown and Person attributes are not yet supported.

kr41/ggrc-core,plamut/ggrc-core,edofic/ggrc-core,VinnieJohns/ggrc-core,VinnieJohns/ggrc-core,plamut/ggrc-core,andrei-karalionak/ggrc-core,josthkko/ggrc-core,j0gurt/ggrc-core,andrei-karalionak/ggrc-core,josthkko/ggrc-core,kr41/ggrc-core,VinnieJohns/ggrc-core,edofic/ggrc-core,selahssea/ggrc-core,j0gurt/ggrc-core,plamut/ggrc-core,selahssea/ggrc-core,AleksNeStu/ggrc-core,andrei-karalionak/ggrc-core,josthkko/ggrc-core,selahssea/ggrc-core,AleksNeStu/ggrc-core,j0gurt/ggrc-core,AleksNeStu/ggrc-core,VinnieJohns/ggrc-core,edofic/ggrc-core,josthkko/ggrc-core,kr41/ggrc-core,AleksNeStu/ggrc-core,edofic/ggrc-core,j0gurt/ggrc-core,plamut/ggrc-core,kr41/ggrc-core,andrei-karalionak/ggrc-core,selahssea/ggrc-core