Additional patient validators and tests

[mbarton]

• Port across the existing tests against the patient model to the patient form where the validators run currently
• Add tests for CSV upload (fixes Add csv upload tests #220)
• Re-arrange the code that modifies patient fields to follow what (I think) is Django best practice:
  • Run them in to_python on a subclass of field if they normalise the data (eg remove spaces from postcode)
  • Run them in clean_XXXX if they only validate a single field
  • Run them in clean if they need to validate a field using the values from others (eg diagnosis date not before dob)
  • Calculate the IMD overriding save on the form rather than on the model
• Only catch RequestException when wrapping remote network validators
  • That way we don't swallow logic errors but do swallow transient network errors
• Mock out all network calls in the unit test
  • I did this originally to mock in return values but it's good practice anyway and speeds up the tests too

This PR adds a couple of behaviour changes:

• Patients can't be older than 25
• Normalise the GP postcode before checking it (fixes Can only look up GP in new patient form if postcode has a space #164)
• Store normalised postcode and GP postcode
• Validate the GP ODS code against the NHS spine
  • Before we just accepted it

[mbarton]

I tried reducing the nesting here with an ExitStack but it didn't work. There's probably a way to do it but it's not that bad and wrapped up in a fixture anyway

[mbarton]

Seen on STAGING (merged by @mbarton 5 minutes and 22 seconds ago) Please check your changes!