Add simple URL-based multireddit title.

r2/r2/models/subreddit.py

@@ -1255,6 +1255,10 @@ class MultiReddit(FakeSubreddit):
         else:
             return FakeSRMember(ModeratorPermissionSet)
 
+    @property
+    def title(self):
+        return _('posts from %s') % ', '.join(sr.name for sr in self.srs)
+
     @property
     def path(self):
         return self._path