Follow-up from "Resolve "API endpoint for exporting project""

The following discussion from gitlab-ce!15860 should be addressed:

• @jameslopez started a discussion: (+3 comments)

  if we want to show the status, ideally, we want to have an export_status similar to import_status. This could also account for scheduled, when we call add_export_job on the project.

  I suggest creating a separate issue for that if you can't tackle this here. Right now, we have to trigger a find on the filesystem to do things like export_project_path. This will also require a further refactor once we implement https://gitlab.com/gitlab-org/gitlab-ee/issues/4157