Explain about text message prefixes when editing

Previously this lead to 2 support tickets because the user didn't
understand why their messages was being split into 2 fragments. We
tried modifying the message about charges, but that made it more
complicated. Adding a hint should hopefully be enough.

app/templates/views/edit-sms-template.html

@@ -15,6 +15,10 @@
       back_link=url_for('main.view_template', service_id=current_service.id, template_id=template.id) if template else url_for('main.choose_template', service_id=current_service.id, template_folder_id=template_folder_id)
     ) }}
 
+    {% if current_service.prefix_sms %}
+      {% set content_hint = 'All messages will start with your service name' %}
+    {% endif %}
+
     {% call form_wrapper() %}
       <div class="govuk-grid-row">
         <div class="govuk-grid-column-two-thirds">
@@ -26,6 +30,7 @@
             form.template_content,
             highlight_placeholders=True,
             width='1-1',
+            hint=content_hint,
             rows=5,
             extra_form_group_classes='govuk-!-margin-bottom-2'
           ) }}

tests/app/main/views/test_templates.py

@@ -1024,6 +1024,30 @@ def test_should_let_letter_contact_block_be_changed_for_the_template(
     )['href'] == expected_partial_url(service_id=SERVICE_ONE_ID)
 
 
+@pytest.mark.parametrize('prefix_sms', [
+    True,
+    pytest.param(False, marks=pytest.mark.xfail())
+])
+def test_should_show_message_with_prefix_hint_if_enabled_for_service(
+    client_request,
+    mocker,
+    mock_get_service_template,
+    mock_get_users_by_service,
+    service_one,
+    fake_uuid,
+    prefix_sms
+):
+    service_one['prefix_sms'] = prefix_sms
+
+    page = client_request.get(
+        '.edit_service_template',
+        service_id=service_one['id'],
+        template_id=fake_uuid,
+    )
+
+    assert 'All messages will start with your service name' in page.text
+
+
 def test_should_show_page_template_with_priority_select_if_platform_admin(
     platform_admin_client,
     platform_admin_user,