simple ui: add back search reference panel

Created by: limitedmage

Closes #40998 (closed)

(review with whitespace changes hidden)

Adds back search reference panel to the sidebar when Simple UI toggle is on. Also fixes the background color of the panel.

Test plan

Verify references panel is shown with Simple UI toggle on and off.

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.