New prescribing data import may miss practices or chemicals

[sebbacon]

There is a constraint on the prescribing table to ensure we have corresponding chemicals and practices.

Since we've started using the detailed prescribing data, this doesn't come with chemicals and practices files, so we'll have to deal with the possibility of missing data another way.

I'm thinking about removing the constraint and adding a report procedure that highlights missing relations for manual intervention.

[sebbacon]

Relates to #367

[sebbacon]

Note that we no longer link directly to the Chemicals table from prescribing, so that particular constraint is no longer relevant.