Tracker favicon requests should be once per session not every announce or scrape

[cfpp2p]

Large trackers can receive millions or requests in a few days for the sites favicon creating unnecessary and unwanted static. i.e. 7,693,320 requests for favicons in 9 days. ​https://forum.transmissionbt.com/viewtopic.php?f=3&t=16798

it's been fixed in the web client but not in Mac, gtk or qt clients.

web client fixed: https://trac.transmissionbt.com/ticket/4549#comment:8

not fixed:

qt client torrent.cc const QString host = Favicons::getHost (QUrl (tracker));

gtk details.c gtk_list_store_set (GTK_LIST_STORE (model), &iter, TRACKER_COL_FAVICON, pixbuf, -1);

Mac TrackerCell?.m #warning better favicon detection

• (void) loadTrackerIcon: (NSString *) baseAddress

related: #5726