Let caseworkers send one off messages

The main task that we think ‘caseworker’ users do is send one off
messages.

So this commit:
- makes sure users who don’t have the `view_activity` permission (ie
  not ‘admin’ users) can still send messages
- adds navigation so that these users have a place to go from which to
  start the process of sending a one off message

tests/app/main/views/test_templates.py

@@ -18,6 +18,7 @@ from tests import (
 )
 from tests.conftest import (
     SERVICE_ONE_ID,
+    active_caseworking_user,
     mock_get_service_email_template,
     mock_get_service_letter_template,
     mock_get_service_template,
@@ -110,6 +111,29 @@ def test_should_show_page_for_one_template(
     mock_get_service_template.assert_called_with(service_one['id'], template_id)
 
 
+def test_caseworker_redirected_to_one_off(
+    client_request,
+    mock_get_service_templates,
+    mocker,
+    fake_uuid,
+):
+
+    mocker.patch('app.user_api_client.get_user', return_value=active_caseworking_user(fake_uuid))
+
+    client_request.get(
+        'main.view_template',
+        service_id=SERVICE_ONE_ID,
+        template_id=fake_uuid,
+        _expected_status=302,
+        _expected_redirect=url_for(
+            'main.send_one_off',
+            service_id=SERVICE_ONE_ID,
+            template_id=fake_uuid,
+            _external=True,
+        ),
+    )
+
+
 @pytest.mark.parametrize('permissions, links_to_be_shown, permissions_warning_to_be_shown', [
     (
         ['view_activity'],