Cache organisation name in Redis

A lot of pages in the admin app are now generated entirely from Redis,
without touching the API.

The one remaining API call that a lot of pages make, when the user is
platform admin or a member of an organisation, is to get the name of
the current service’s organisation.

This commit adds some code to start caching that as well, which should
speed up page load times for when we’re clicking around the admin app
(it’s typically 100ms just to get the organisation, and more than that
when the API is under load).

This means changing the service model to get the organisation from the
API by ID, not by service ID. Otherwise it would be very hard to clear
the cache if the name of the organisation ever changed.

We can’t cache the whole organisation because it has a
`count_of_live_services` field which can change at any time, without an
update being made.

tests/app/main/views/service_settings/test_service_setting_permissions.py

@@ -1,10 +1,11 @@
 import functools
+from unittest.mock import PropertyMock
 
 import pytest
 from flask import url_for
 
 from app.main.views.service_settings import PLATFORM_ADMIN_SERVICE_PERMISSIONS
-from tests.conftest import normalize_spaces
+from tests.conftest import ORGANISATION_ID, normalize_spaces
 
 
 @pytest.fixture
@@ -14,7 +15,7 @@ def get_service_settings_page(
     service_one,
     mock_get_inbound_number_for_service,
     mock_get_all_letter_branding,
-    mock_get_service_organisation,
+    mock_get_organisation,
     mock_get_free_sms_fragment_limit,
     no_reply_to_email_addresses,
     no_letter_contact_blocks,
@@ -76,9 +77,22 @@ def test_service_set_permission(
     ({'permissions': ['letter']},
      '.service_set_permission', {'permission': 'international_letters'}, 'Send international letters Off Change'),
 ])
-def test_service_setting_toggles_show(get_service_settings_page, service_one, service_fields, endpoint, kwargs, text):
+def test_service_setting_toggles_show(
+    mocker,
+    get_service_settings_page,
+    service_one,
+    service_fields,
+    endpoint,
+    kwargs,
+    text,
+):
     link_url = url_for(endpoint, **kwargs, service_id=service_one['id'])
     service_one.update(service_fields)
+    mocker.patch(
+        'app.models.service.Service.organisation_id',
+        new_callable=PropertyMock,
+        return_value=ORGANISATION_ID,
+    )
     page = get_service_settings_page()
     assert normalize_spaces(page.find('a', {'href': link_url}).find_parent('tr').text.strip()) == text
 
@@ -145,7 +159,7 @@ def test_normal_user_doesnt_see_any_platform_admin_settings(
     service_one,
     no_reply_to_email_addresses,
     no_letter_contact_blocks,
-    mock_get_service_organisation,
+    mock_get_organisation,
     single_sms_sender,
     mock_get_all_letter_branding,
     mock_get_inbound_number_for_service,