Design templates for common page types [headers and breadcrumbs]

Created by: christinaforney

Campaigns and Insights have different header implementations, though they live at the same level. Create a header template design for all non-search pages. A React layout component will be developed from this design to help improve consistency.

• https://sourcegraph.com/insights
• https://sourcegraph.com/campaigns
• https://sourcegraph.com/extensions

This project should address common header elements, breadcrumbs, and other features that should be standardized across all non-search pages.