Fix Import/Export foreign key issue to do with project members

Cleans-up any foreign keys in `ProjectMember` - same as we do with the rest of the models when importing.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/23837 and https://gitlab.com/gitlab-org/gitlab-ce/issues/23739

See merge request !2020
Signed-off-by: Rémy Coutable <remy@rymai.me>