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
- [-] 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