Add ‘GP’ as an org 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

tests/app/organisation/test_rest.py

@@ -204,7 +204,10 @@ def test_post_create_organisation_existing_name_raises_400(admin_request, sample
         'name': 'Service name',
         'crown': False,
         'organisation_type': 'foo',
-    }, 'organisation_type foo is not one of [central, local, nhs_central, nhs_local, emergency_service, school_or_college, other]'),  # noqa
+    }, (
+        'organisation_type foo is not one of '
+        '[central, local, nhs_central, nhs_local, gp, emergency_service, school_or_college, other]'
+    )),
 ))
 def test_post_create_organisation_with_missing_data_gives_validation_error(
     admin_request,