consider using blob URLs for favicons

[tanriol]

data: URLs are expensive, and sometimes not in nice places. For example, it was found that a feedlist build and the corresponding favicon URL assignment can lock the parent Firefox process for multiple seconds as it receives the URLs and checks them against add-on webRequest and webNavigation handlers if one has add-ons that register handlers for the affected hooks.

This is partially related to #466, but can be much more severe.