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.

tests/app/main/views/test_add_service.py

@@ -55,7 +55,7 @@ def test_get_should_render_add_service_template(
         'local',
         'nhs_central',
         'nhs_local',
-        'nhs_local',
+        'nhs_gp',
         'emergency_service',
         'school_or_college',
         'other',
@@ -205,7 +205,7 @@ def test_get_should_only_show_nhs_org_types_radios_if_user_has_nhs_email(
     ] == [
         'nhs_central',
         'nhs_local',
-        'nhs_local',
+        'nhs_gp',
     ]