Create variables for application header, footer, and viewport heights

What does this MR do and why?

• $calc-application-header-height: system header, performance bar, and header when present
• $calc-application-footer-height system footer when present
• $calc-application-viewport-height full height minus system header, performance bar, header, and/or system footer when present
• Update existing top/bottom/height usage with variables
• Remove explicit .with-performance-bar, .with-system-header, and .with-system-footer styles
• Move .fixed-top style override to utilties.scss
• Remove content-wrapper-margin styles and class usage (as per #332705 (closed))
  • Instead use layout-page with padding-top/padding-bottom for fixed elements on any given page

Screenshots or screen recordings

Before	After
CleanShot_2023-03-21_at_11.50.22	CleanShot_2023-03-21_at_12.11.43
top-position-before	top-position-after

How to set up and validate locally

Add system header/footer messages in admin settings and review views:

• Sign in http://127.0.0.1:3000/users/sign_in
• Merge request (default) http://127.0.0.1:3000/flightjs/Flight/-/merge_requests/4/diffs
• Merge request with sticky header Feature.enable(:moved_mr_sidebar) http://127.0.0.1:3000/flightjs/Flight/-/merge_requests/4/diffs
• Issue http://127.0.0.1:3000/flightjs/Flight/-/issues/33
  • Add design and open design to view fixed overlay
• Wiki create diagram http://127.0.0.1:3000/flightjs/Flight/-/wikis/home?view=create (added in !112569 (merged))
• Job with long text and sticky header within output http://127.0.0.1:3000/flightjs/Flight/-/jobs/675
• Board http://127.0.0.1:3000/groups/flightjs/-/boards
  • Open board issue item and view drawer

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.