Show indicator in the UI that code search is served from Zoekt

Currently our UI only shows "Advanced Search is enabled" with a link to the advanced search docs. There should be some indicator in the UI that search results are actually served by exact code search with the correct docs link. This will help with support and debugging.

If use_zoekt? is true, we want to say Exact Code Search (powered by Zoekt) is enabled with a link to zoekt help page.