Add platform admin button to enable caseworking

Eventually the ability to have the caseworking view for your service
will be self-service. But while we’re still building it only we should
be able to enable it, for our own testing purposes.

So this commit adds platform admin functionality to switch the feature
on and off for our services.

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

@@ -45,6 +45,11 @@ def get_service_settings_page(
     ({'permissions': []},
         '.service_switch_can_upload_document', {}, 'Allow to upload documents'),
 
+    ({'permissions': []},
+        '.service_switch_caseworking', {}, 'Allow granting of caseworking permission'),
+    ({'permissions': ['caseworking']},
+        '.service_switch_caseworking', {}, 'Stop granting of caseworking permission'),
+
     ({'permissions': ['sms']}, '.service_set_inbound_number', {'set_inbound_sms': True}, 'Allow inbound sms'),
 
     ({'active': True}, '.archive_service', {}, 'Archive service'),