Backend: Allow Group Maintainers to manage Group Variables

Summary

Maintainers can no longer access Group CI/CD variables.

Steps to reproduce

Set yourself to a maintainer of a group. Visit that group's page, note that CI/CD settings are MIA.

Example Project

This is happening on the current version at the time of this writing; 12.5

What is the current bug behavior?

The page link is NOT there.

What is the expected correct behavior?

• Pulling from the GitLab docs, here: https://docs.gitlab.com/ee/user/permissions.html
• The only time that "Manage Variables" is checked, it's checked for BOTH Maintainers & Owners.

I get the notion that the lack of "manage variable" documentation around this difference in groups suggest that it's not meant to be this way.

Possible fixes

Update the linked docs or update the application to reflect what the docs suggest.