Merge branch 'fix/import-export-retry-settings' into 'master'

Limit number of project export retries to 3

I think 3 should be a sufficient number of retries - at the moment we were doing 20+

This will only send a maximum of 3 emails if the project export went wrong. Tested - project import is not affected.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/18966

See merge request !5267