Fix 500 error on POST to /questionnaire/relationships (#461)

Sending a POST request to `/questionnaire/relationships` was failing
because it was falling into the non-relationship block handling code.
Although we wouldn't expect this behaviour, it has been observed
during testing, and should be handled properly. To fix this `POST`
has been enabled on the `/questionnaire/relationships` endpoint, which
consequently does the same as the `GET` and redirects to the first
relationship.

Author: ajmaddaford

app/routes/questionnaire.py

@@ -220,7 +220,7 @@ def block(schema, questionnaire_store, block_id, list_name=None, list_item_id=No
 
 @questionnaire_blueprint.route(
     "relationships/",
-    methods=["GET"],
+    methods=["GET", "POST"],
 )
 @questionnaire_blueprint.route(
     "relationships/<list_name>/<list_item_id>/to/<to_list_item_id>/",

tests/integration/questionnaire/test_questionnaire_relationships.py

@@ -118,3 +118,11 @@ def test_relationship_not_altered_when_new_list_item_not_submitted(self):
         self.post({"anyone-else": "No"})
 
         self.assertEqual(list_item_ids_original, list_item_ids_new)
+
+    def test_post_to_relationships_root(self):
+        self.launchSurvey("test_relationships")
+        self.add_person("Marie", "Doe")
+        self.add_person("John", "Doe")
+        self.post({"anyone-else": "No"})
+        self.post(url="/questionnaire/relationships")
+        self.assertStatusOK()