Add endpoint for getting / creating a hidden template

We want a way of getting the hidden precompiled template from admin,
so this adds an endpoint which gets the template or creates it if it doesn't
exist. The function to get or create the hidden template already existed
but has been moved to the template DAO now that it is used in more
places.

tests/app/template/test_rest.py

@@ -389,6 +389,50 @@ def test_should_be_able_to_archive_template(client, sample_template):
     assert Template.query.first().archived
 
 
+def test_get_precompiled_template_for_service(
+    client,
+    notify_user,
+    sample_service,
+):
+    assert len(sample_service.templates) == 0
+
+    response = client.get(
+        '/service/{}/template/precompiled'.format(sample_service.id),
+        headers=[create_authorization_header()],
+    )
+    assert response.status_code == 200
+    assert len(sample_service.templates) == 1
+
+    data = json.loads(response.get_data(as_text=True))
+    assert data['name'] == 'Pre-compiled PDF'
+    assert data['hidden'] is True
+
+
+def test_get_precompiled_template_for_service_when_service_has_existing_precompiled_template(
+    client,
+    notify_user,
+    sample_service,
+):
+    create_template(
+        sample_service,
+        template_name='Exisiting precompiled template',
+        template_type=LETTER_TYPE,
+        hidden=True)
+    assert len(sample_service.templates) == 1
+
+    response = client.get(
+        '/service/{}/template/precompiled'.format(sample_service.id),
+        headers=[create_authorization_header()],
+    )
+
+    assert response.status_code == 200
+    assert len(sample_service.templates) == 1
+
+    data = json.loads(response.get_data(as_text=True))
+    assert data['name'] == 'Exisiting precompiled template'
+    assert data['hidden'] is True
+
+
 def test_should_be_able_to_get_all_templates_for_a_service(client, sample_user, sample_service):
     data = {
         'name': 'my template 1',