(Part 2) FE multiple approval rules - settings full

What does this MR do?

This MR implements the FE components for multiple approval rule settings.

Please note! To make this MR smaller, the Vuex part has been split into a separate MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9043, but this MR still contains the functionality in a merge commit.

What will be handled in a separate MR?

• Spec for approvers_select.
• License feature with only a single approval rule.

Instructions

• This does not point to master. We will merge the FE branch once the BE is merged.
• This is behind the feature flag approval_rule. You will need to enable this feature locally to test.
• Do not merge until Part 3 !9043 (merged) has been merged and the related merge commit has been rebased out.
• Please merge the CE MRs to CE master first.

CE MR's

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24139 for user-avatar-list
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24140 for gl-modal-vuex
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24141 for shared css updates

Screenshots

Demo	Mobile View List	Mobile View Form

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