Add private code CTAs
Created by: attfarhan
Fixes https://github.com/sourcegraph/sourcegraph/issues/11799.
Adds CTAs to install Sourcegraph for private code to search homepage, campaigns, and repogroup pages, as in the design: https://www.figma.com/file/X5WmkZmMv8wACPpxFDgKpX/RFC-165-Search-CTAs?node-id=0%3A1&viewport=2658%2C620%2C0.25