Serialise content for broadcast templates

Broadcast services only have broadcast templates. But on the frontend
we show the template type under the name of the template. This is
redundant. It would be better to preview the content of the template
instead.

At the moment we can’t do this because we optimised the template schema
response to not include the content of the templates in
https://github.com/alphagov/notifications-api/pull/2880

This commit reverses that optimisation, for broadcast templates only,
and expands the tests to cover all template types.

tests/app/template/test_rest.py

@@ -561,22 +561,37 @@ def test_should_get_return_all_fields_by_default(
     {'detailed': False},
     {'detailed': 'False'},
 ))
+@pytest.mark.parametrize('template_type, expected_content', (
+    (EMAIL_TYPE, None),
+    (SMS_TYPE, None),
+    (LETTER_TYPE, None),
+    (BROADCAST_TYPE, 'This is a test'),
+))
 def test_should_not_return_content_and_subject_if_requested(
     admin_request,
-    sample_email_template,
+    sample_service,
     extra_args,
+    template_type,
+    expected_content,
 ):
+    create_template(
+        sample_service,
+        template_type=template_type,
+        content='This is a test',
+    )
     json_response = admin_request.get(
         'template.get_all_templates_for_service',
-        service_id=sample_email_template.service.id,
+        service_id=sample_service.id,
         **extra_args
     )
     assert json_response['data'][0].keys() == {
+        'content',
         'folder',
         'id',
         'name',
         'template_type',
     }
+    assert json_response['data'][0]['content'] == expected_content
 
 
 @pytest.mark.parametrize(