Look at service’s organisation on agreement page

Usually the service’s organisation and the user’s current organisation
will be the same. But this won’t be the case when:
- someone with a non-government email address is looking at the page
- someone from our team, as a platform admin user, is looking at the
  page (it will show Cabinet Office instead)

This commit fixes these problems by explicitly looking at the service’s
organisation. We couldn’t do this previously because when this page
wasn’t service-specific `current_service` was not guaranteed to be set.

tests/app/main/views/test_agreement.py

@@ -4,7 +4,11 @@ from io import BytesIO
 import pytest
 from flask import url_for
 
-from tests.conftest import SERVICE_ONE_ID, mock_get_organisation_by_domain
+from tests.conftest import (
+    SERVICE_ONE_ID,
+    mock_get_organisation_by_domain,
+    mock_get_service_organisation,
+)
 
 
 class _MockS3Object():
@@ -16,16 +20,18 @@ class _MockS3Object():
         return {'Body': BytesIO(self.data)}
 
 
-@pytest.mark.parametrize('endpoint, extra_args, link_selector, expected_back_links', [
+@pytest.mark.parametrize('endpoint, extra_args, organisation_mock, link_selector, expected_back_links', [
     (
         'main.agreement',
         {},
+        mock_get_organisation_by_domain,
         'main .column-two-thirds a',
         []
     ),
     (
         'main.service_agreement',
         {'service_id': SERVICE_ONE_ID},
+        mock_get_service_organisation,
         'main .column-five-sixths a',
         [
             partial(url_for, 'main.request_to_go_live', service_id=SERVICE_ONE_ID)
@@ -66,10 +72,11 @@ def test_show_agreement_page(
     expected_links,
     endpoint,
     extra_args,
+    organisation_mock,
     link_selector,
     expected_back_links,
 ):
-    mock_get_organisation_by_domain(
+    organisation_mock(
         mocker,
         crown=crown,
         agreement_signed=agreement_signed,