Support restricting group access by multiple IP subnets

Problem

We delivered on https://gitlab.com/gitlab-org/gitlab-ee/issues/1985, but this iteration was restricted to a single subnet. For organizations distributed over multiple locations, supporting multiple subnets is a hard requirement.

Proposal

• Allow multiple subnets to be defined in our group-level IP address access restriction.
  • For this iteration, users should be able to specify multiple subnets using commas as a delimiter.