Add Relatable mixin to Document model
Add Relatable mixin to Document model

Since Document model is linked to other models via relationships table,
it needs a Relatable mixin otherwise the related_object property does
not work.

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