Adding some endpoints for handling consents belonging to a person

I want to be able to add some consents to existing guests and added some endpoints in the Greg API for doing that.

greg/tests/api/test_person.py

@@ -537,7 +537,7 @@ def test_person_consent_add(
 def test_person_consent_patch(
     client: APIClient, person: Person, consent_fixture_choice_yes: Consent
 ):
-    consent_id = person.consents.first().id
+    consent_id = person.consents.get().id
     url = reverse(
         "v1:person_consent-detail", kwargs={"person_id": person.id, "id": consent_id}
     )
@@ -559,7 +559,7 @@ def test_person_consent_patch(
 def test_person_consent_delete(
     client: APIClient, person: Person, consent_fixture_choice_yes: Consent
 ):
-    consent_id = person.consents.first().id
+    consent_id = person.consents.get().id
     get_url = reverse("v1:person_consent-list", kwargs={"person_id": person.id})
 
     consents_for_person = client.get(get_url).json()["results"]