Refactor positions and widths for fixed application bars

What does this MR do and why?

Refactor left/right positions for application bars

• Remove --width custom property and manual calcuations for combinations of left and right sidebar widths
• Use width: auto combined with left/right to set application bar postions and widths with --application-bar-left and --application-bar-right custom properties
• Move setting of custom properties to common.scss inline with .with-* classes used for header height calculations

Screenshots or screen recordings

New Navigation

Feature.disable(:moved_mr_sidebar)

Screenshots

Viewport	State	Screenshot
Large	All expanded
Large	Right collapsed
Large	All collapsed
Medium	Right expanded
Medium	Right collapsed
Small	Right expanded
Small	Right collapsed

Feature.enable(:moved_mr_sidebar)

Screenshots

Viewport	State	Screenshot
Large	All expanded
Large	All collapsed
Medium	Right expanded
Medium	Right collapsed
Small	Right expanded
Small	Right collapsed

Old Navigation

Feature.disable(:moved_mr_sidebar)

Screenshots

Viewport	State	Screenshot
Large	All expanded
Large	Right collapsed
Large	All collapsed
Medium	Right expanded
Medium	Right collapsed
Small	Right expanded
Small	Right collapsed

Feature.enable(:moved_mr_sidebar)

Screenshots

Viewport	State	Screenshot
Large	All expanded
Large	All collapsed
Medium	Right expanded
Medium	Right collapsed
Small	Right expanded
Small	Right collapsed

How to set up and validate locally

1. Review merge request view:
   1. with left and right sidebars collapsed and expanded
   2. at small (<768px), medium (<1200px), and large (>=1200px) viewports
   3. with :moved_mr_sidebar feature flag toggled on and off
   4. with new navigation toggled on and off

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.

Related to #410016 (closed)