Remove `datetime` from Prediction model

And change all views, commands, tests etc to use the new 
Prediction.minute.datetime instead.

The first migration checks that all Predictions have a `minute` field which
matches their `datetime` field. It then removes the `datetime` field.

The second migration makes `minute` compulsory.

https://www.pivotaltracker.com/story/show/76374644
[#76374644]