(meta) Organize and Standardize settings

Goal of these issues are to clean up our overall approach to Settings, standardize the pages, and make the navigation more standard with tabs, etc.

• Suggestion: Replace settings gear in projects/etc. with a tab: https://gitlab.com/gitlab-org/gitlab-ce/issues/23007
• Replace Admin Settings with a tab: https://gitlab.com/gitlab-org/gitlab-ce/issues/23303 (Waiting on !6999 (closed) to un-WIP)
• Replace Group Settings with a tab: https://gitlab.com/gitlab-org/gitlab-ce/issues/23304 (Waiting on !6999 (closed) to un-WIP)
• 'Leave Project' and 'Leave Group' should be buttons: https://gitlab.com/gitlab-org/gitlab-ce/issues/23305
• Standardize the Settings pages views https://gitlab.com/gitlab-org/gitlab-ce/issues/22210 (Awaiting UX)

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Related to (meta) Give love to settings screens #22171. However, this issue is more about the overall structure and layout to settings, and that issue digs into details on various settings pages.