web: code intel panel header + references section redesign

Created by: valerybugakov

Description

The redesign of the Code Intel bottom panel header and references section: https://github.com/sourcegraph/sourcegraph/issues/21045. Figma mockups.

Changes

• Added redesign styles to the panel header.
• Added redesign styles to the references panel.
• Extracted the HierarchicalLocationsViewButton component.
• Added minor tweaks to the tabs components to match redesign styles.
• Fixed file match icon size.
• Fixed margin between file matches to be 16px.
• Added basic styles to the resizable handler.

The resizable handle, empty and loading states will be updated in a separate PR.

Screenshots

Before:

After: