RSS: prevent fetching of channel icons

As mentioned in ticket:19031#comment:4 Thunderbird fetches at each start the icons of each rss channel, to prevent Thunderbird from fetching all RSS channel icons at start/simultaneously (as torbirdy prevents TB to fetch emails at start) I suggest torbirdy shouldn't fetch them at all. (fetching all icons at the same time might identify a user)

browser.chrome.site_icons = false