Add link to 'search in Sourcegraph' to code host search pages

Created by: christinaforney

From a search page, or when search is interacted with, show a link for the user to go search in Sourcegraph instead. We can even parse the current search query and use that to populate the search in Sourcegraph.

Including sourcegraph “banner ads” at the top of Github Search by running the query from the URL through the sourcegraph search engine and embedding your results when the plugin is installed: https://github.example.com/search?q=SourcegraphIsAwesome

Requested by a user at https://app.hubspot.com/contacts/2762526/company/557691596