Sort letter brandings alphabetically

Currently the brandings have non-deterministic sorting, which means
the order changes from page load to page load. This makes it hard to
find the item you’re looking for.

This commit sorts them by the name of the branding, same as for email
brandings.

tests/app/main/views/test_service_settings.py

@@ -2088,6 +2088,7 @@ def test_set_letter_branding_platform_admin_only(
 @pytest.mark.parametrize('current_dvla_org_id, expected_selected', [
     (None, '001'),
     ('500', '500'),
+    ('999', '999'),
 ])
 def test_set_letter_branding_prepopulates(
     logged_in_platform_admin_client,
@@ -2101,6 +2102,20 @@ def test_set_letter_branding_prepopulates(
     response = logged_in_platform_admin_client.get(url_for('main.set_letter_branding', service_id=service_one['id']))
     assert response.status_code == 200
     page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
+
+    for element in {'label', 'input[type=radio]'}:
+        assert len(page.select(element)) == 3
+
+    assert normalize_spaces(page.select('label')[0].text) == 'Animal and Plant Health Agency'
+    assert page.select('input')[0]['value'] == '999'
+
+    assert normalize_spaces(page.select('label')[1].text) == 'HM Government'
+    assert page.select('input')[1]['value'] == '001'
+
+    assert normalize_spaces(page.select('label')[2].text) == 'Land Registry'
+    assert page.select('input')[2]['value'] == '500'
+
+    assert len(page.select('input[checked]')) == 1
     assert page.select('input[checked]')[0]['value'] == expected_selected