Add Outbound requests whitelist for local networks

What does this MR do?

This MR is based on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21377

This MR adds a whitelist that can be used to allow requests to certain IPs/domains on the local network when requests to the local network from hooks and services are disabled.

It allows the following types of entries in the whitelist:

• Domains: domain.com, subdomain.domain.com
• IPv4 and IPv6: 192.168.1.1, fe80::c800:eff:fe74:8
• IP ranges: 127.0.0.0/28, 1:0:0:0:0:0:0:0/124

The following are not supported:

• Domain wildcards: *.domain.com
• Ports: 192.168.1.1:3000 (Do we need to support this?)

Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/44496

Putting unicode values or HTML tags in the whitelist causes an error:

whitelist_validations

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers