Modify unique token to use url safe characters.

Looks like the merge of the methods I chose cause url unsafe / to be
emitted sometimes. This causes poe post to fail as the url includes
the key and the / messes up the rest route url parsing code.

Author: rouilj

roundup/backends/sessions_common.py

@@ -41,10 +41,10 @@ def get_logger(self):
         return self.__logger
 
     def getUniqueKey(self, length=40):
-        otk = b2s(base64.b64encode(
+        otk = b2s(base64.urlsafe_b64encode(
             random_.token_bytes(length))).rstrip('=')
         while self.exists(otk):
-            otk = b2s(base64.b64encode(
+            otk = b2s(base64.urlsafe_b64encode(
                 random_.token_bytes(length))).rstrip('=')
 
         return otk