add optional nginx configs to make more secure

Add back some items removed in https://github.com/gitlabhq/gitlabhq/commit/8aff39a6d0a63e904ada910900ee07a1c3190a9d but makes them optional so they can be ignored if you want a simple setup experience.
@bbodenmiller I know @RaymiiOrg personally and value his opinion. However, I still think this section should not be recommended.