Add ‘GP’ as an organisation type

Although their allowances are the same as what we call `nhs_local` it
makes more sense to store them separately because:

- we already present them as two separate choices to the user
- we may want to handle them differently in the future, eg in terms of
  what branding choices are available to them

Once the API is updated we can start passing in this new value from
the admin app.

app/models/organisation.py

@@ -12,7 +12,7 @@ class Organisation(JSONModel):
         ('local', 'Local government'),
         ('nhs_central', 'NHS – central government agency or public body'),
         ('nhs_local', 'NHS Trust or Clinical Commissioning Group'),
-        ('nhs_local', 'GP practice'),
+        ('nhs_gp', 'GP practice'),
         ('emergency_service', 'Emergency service'),
         ('school_or_college', 'School or college'),
         ('other', 'Other'),