Remotely fetched favicons must not be larger than 10KB (see bug 361923).
Due to the age of the mentioned bug (since 2006-11-26) and since it seems that the limit of 10 000 B is intentional
intentionality putting a limit to the icon file to void the search.json file to turn too heavy
which both imply it's not going to be solved soon, we should provide either of following
- smaller icons for favicon
- separate icons for opensearch
- dataURI encoded icons in opensearch modules