Fix export project file permissions issue

Fixes security concerns of https://gitlab.com/gitlab-org/gitlab-ce/issues/22757

I have just added the permissions 0700 to the creation of any of the export paths, as @jacobvosmaer suggested in https://gitlab.com/gitlab-org/gitlab-ce/issues/22757#note_16197616

After this has fixed, it could take up to 24 hours in the worse case scenario for old archives to be completely safe - This is the time `ImportExportProjectCleanupWorker` may take to remove the folders. The temporary folders will be 0700 straight away for new installations.

See merge request !2003
Signed-off-by: Rémy Coutable <remy@rymai.me>