Rethink backend Arima view

The current backend Arima view (schools/schooldID/predictions/) makes predictions starting from the last timestamp of the accounting data for that school. If a user accidentally uploads data with a date too far into the future this will mess up this logic. Can either think of a new way of doing predictions (i.e. allow more than 12 values etc etc.), or just add more strict upload filter/validation based on current date.