Add unit test for verifying replies are gone when source is deleted
Add unit test for verifying replies are gone when source is deleted

conorsch/securedrop,ageis/securedrop,ageis/securedrop,conorsch/securedrop,garrettr/securedrop,conorsch/securedrop,ehartsuyker/securedrop,ageis/securedrop,heartsucker/securedrop,micahflee/securedrop,conorsch/securedrop,ehartsuyker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,micahflee/securedrop,ehartsuyker/securedrop,micahflee/securedrop,heartsucker/securedrop,micahflee/securedrop,ehartsuyker/securedrop,garrettr/securedrop,garrettr/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,ageis/securedrop,heartsucker/securedrop,garrettr/securedrop