Make Import/Export rate limits configurable application settings

Currently Import/Export Rate Limits are hardcoded here: https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/application_rate_limiter.rb#L23-32

These figures are based on what is acceptable for GitLab.com, but might be too restrictive (or too loose) for self-managed installations.

Similar to Raw Blob endpoint rate limits, make them configurable application settings https://docs.gitlab.com/ee/user/admin_area/settings/rate_limits_on_raw_endpoints.html that can be adjusted by instance admins.

This also greatly benefits ProServ team when migrating to and from self-managed instances, where these limitations don't have to be in place.