Due to a serious security issue with the new Flask notebook (see
#13270), it has been decided on that ticket not to ship sage-5.2 with
sagenb-0.9.0.
Since I see no reason why the other tickets in sage-5.2 couldn't be
released, I will unmerge #11080 and related tickets and release sage-5.2
still with the old notebook.
If you don't care about #13270, feel free to use sage-5.2.rc0:
http://boxen.math.washington.edu/home/release/sage-5.2.rc0/
It's a pity that this has to happen, but we need to move on,
Jeroen.