Refactor CSS and move it to it's own CSS file

What does this MR do and why?

Refactors CSS to use Tailwind wherever possible and restructures CSS into two blocks:

• Application chrome
• Paneled layout
• Moves CSS to it's own file (application-chrome.scss)
• Includes a gap fix between the super sidebar and the first panel
• Includes a top spacing fix for the super sidebar
• Refines the panel header horizontal spacing
• Fix GlDrawer height

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Related to #548836 (closed)