Add a feature flag for Feature Flag index page TAB change

What does this MR do?

This MR adds a feature flag to https://gitlab.com/gitlab-org/gitlab-ee/issues/7731. So that we can slack off to merge https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8821 without worrying about the feature freezing day.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• 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