Export designs when creating a project export

Right now, the database model, nor the design repository are included in an export. Which means they would be lost.

Ideally we'd include the design repository in the export and when the model is included designs should be available again after an import

The existing LFS export and import classes will also need to be modified. See https://gitlab.com/gitlab-org/gitlab-ee/issues/11090#note_188616511

Documentation should be updated https://docs.gitlab.com/ee/user/project/settings/import_export.html