(Part 1) FE multiple approval rules - settings empty state

(CE Port https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24087)

What does this MR do?

This MR sets up the structure for the multiple approval rules settings. To keep this MR focused, it only shows the empty state. The full functionality will be implemented by upcoming MRs.

Please Note!

• This does not point to master. We will merge the FE branch once the BE is merged. We'll go ahead and merge the backstage CE port to master.
• This is behind the feature flag approval_rule. You will need to enable this feature locally to test.
• There is no integration with the BE. This integration will be merged in a separate MR. See branch 1979-fe-settings-integrated.

Screenshots

Feature.enable(:approval_rule)	Feature.disable(:approval_rule) (default)

What are the relevant issue numbers?

• #1979 (closed)
• FE #8350 (closed)

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