Add Reminderable mixin
Add Reminderable mixin

Adds reminderable mixin that should be used for various reminder.
At the moment only one handler (sending reminder based on object
state) is provided.

edofic/ggrc-core,plamut/ggrc-core,selahssea/ggrc-core,selahssea/ggrc-core,josthkko/ggrc-core,selahssea/ggrc-core,NejcZupec/ggrc-core,andrei-karalionak/ggrc-core,josthkko/ggrc-core,kr41/ggrc-core,j0gurt/ggrc-core,kr41/ggrc-core,prasannav7/ggrc-core,selahssea/ggrc-core,kr41/ggrc-core,VinnieJohns/ggrc-core,prasannav7/ggrc-core,NejcZupec/ggrc-core,AleksNeStu/ggrc-core,edofic/ggrc-core,andrei-karalionak/ggrc-core,AleksNeStu/ggrc-core,NejcZupec/ggrc-core,plamut/ggrc-core,andrei-karalionak/ggrc-core,prasannav7/ggrc-core,AleksNeStu/ggrc-core,VinnieJohns/ggrc-core,AleksNeStu/ggrc-core,j0gurt/ggrc-core,j0gurt/ggrc-core,josthkko/ggrc-core,edofic/ggrc-core,VinnieJohns/ggrc-core,VinnieJohns/ggrc-core,NejcZupec/ggrc-core,j0gurt/ggrc-core,edofic/ggrc-core,andrei-karalionak/ggrc-core,plamut/ggrc-core,kr41/ggrc-core,josthkko/ggrc-core,plamut/ggrc-core,prasannav7/ggrc-core