[flickr] restore image quality

Flickr started serving images from live.staticflickr.com (see ec88ff1),
but the old farmN.staticflickr.com URLs still work - at least for the
time being.
Filesize (and most likely quality as well) for images from live.…  is
severely reduced compared to images from farmN.… for non-original files,
so all live URLs are replaced to point to a randomly chosen farm server.

gallery_dl/extractor/flickr.py

@@ -10,6 +10,7 @@
 
 from .common import Extractor, Message
 from .. import text, oauth, util, exception
+import random
 
 
 class FlickrExtractor(Extractor):
@@ -25,12 +26,13 @@ class FlickrExtractor(Extractor):
         self.load_extra = self.config("metadata", False)
 
     def items(self):
+        farm = "//farm{}.".format(random.randint(1, 9))
         info = self.data()
         yield Message.Version, 1
         yield Message.Directory, info
         for photo in self.photos():
             photo.update(info)
-            url = photo["photo"]["source"]
+            url = photo["photo"]["source"].replace("//live.", farm)
             yield Message.Url, url, text.nameext_from_url(url, photo)
 
     def data(self):
@@ -94,7 +96,8 @@ class FlickrImageExtractor(FlickrExtractor):
             info = {"id": self.item_id}
 
         info["photo"] = size
-        url = size["source"]
+        farm = "//farm{}.".format(random.randint(1, 9))
+        url = size["source"].replace("//live.", farm)
         text.nameext_from_url(url, info)
 
         yield Message.Version, 1