Remove `datetime` from Prediction model

**NOTE: Before deploying this you must have run the `add_minute_for_each_prediction` to create `Minute` objects for each `Prediction`**

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]