Search: Add affiliate program support

We have a "Search powered by Jetpack" link option in the sidebar. We could let end users add their affiliate information to that link: https://refer.wordpress.com/

This would give them an extra incentive to use the program.

cc @robertbpugh

Start by locating the sidebar option that renders the “Search powered by Jetpack” link and inspect how it currently uses https://refer.wordpress.com/. Determine how end users would provide affiliate information and verify that the link includes it when configured; the issue does not name a file or test.

Written by the indexing model from the issue text.