Change django message storage backends default.

Due to original cookie storage backends have limit 2048 bytes,
change it to fallbackstorage, which could store extra message in
session storage.

Related django storage illustration
https://docs.djangoproject.com/en/1.6/ref/contrib/messages/#module-django.contrib.messages

Closes-Bug: #1168541
Change-Id: Ieed4db962ccdadcb346c8ca7e6e789766f9838e5
Merge "Change django message storage backends default."