Resolve "IP whitelisting for Geo-enabling functionality in the primary"

What does this MR do?

Currently, Geo user authentication, file and repository synchronization works by having the secondary connect to the primary with privileged functionality, mostly mediated via JSON web tokens. However, we allow those connections to come from any IP in the world. By this MR we add the ability to specify allowed IP and CIDRs.

Backport MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26930

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/4193

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

Closes #4193 (closed)