Add Unknown option to Calcineurin Toxicity question

[marshalc]

Request to expand the list of options for FB24 Calcineurin inhibitor from Yes/No/Unanswered, to Yes/No/Unknown/Unanswered - made at WP4 Data Cleanup meeting of 13-03-2018@2pm

This means a changing the field from a NullBooleanField to a PositiveSmallIntegerField and mapping the current data to the new equivalent value.

[marshalc]

FB24 may be a moot point because there's a query for each about if a biopsy was not take, then the answer would be no.

Other questions:

• Is it logistically possible to measure the PO2 perfusate - pragmatically if there is no data already for it, then it is near impossible to chase up, so this can be left as no by default.