Move global search left rail items to new left sidebar

Details

On the global search results page, we should utilize the left sidebar for the scopes and filters/facets that are currently displayed within a left rail on the page. This allows us to maintain the global navigation while keeping the filtering options co-located within a shared region of the page.

Implementation Guide

Resources

• Figma Link
• Prototype

Tasks

• Relocate the scopes and filters/facets that are presented in the left rail into the new left sidebar
• The context switcher title should read "Search results"
• The context switcher icon should use the finalized icon from gitlab-svgs#340 (closed)
• For mobile/smaller screen sizes, display a "Filter" button above the results that toggles the sidebar of Filter options
• Update the "Empty" state of global search (navigating to /search) to display the scope options. Since there would be no Group or Project filters applied yet, only show the scopes that we display for the global level (example below – though this wouldn't include counts). Handled in #392835 (closed)