Allows services to choose if they can send letters

Our support ticket analysis shows that the most common action request
after going live is turning on letters.

We just do this for any team that requests it – there’s no gatekeeping.
So we should just allow people to make the change themselves.

This will be a better experience for our users, and less work for us.
The design of the page replicates roughly what we have for international
text messaging.

tests/app/main/views/test_service_settings.py

@@ -1557,40 +1557,50 @@ def test_should_set_sms_allowance(
     )
 
 
+@pytest.mark.parametrize((
+    'expected_initial_value,'
+    'posted_value,'
+    'initial_permissions,'
+    'expected_updated_permissions'
+), [
+    ('off', 'on', ['email', 'sms'], ['email', 'sms', 'letter']),
+    ('on', 'off', ['email', 'sms', 'letter'], ['email', 'sms']),
+])
 def test_switch_service_enable_letters(
-    logged_in_platform_admin_client,
+    client_request,
     service_one,
     mocker,
+    expected_initial_value,
+    posted_value,
+    initial_permissions,
+    expected_updated_permissions,
 ):
     mocked_fn = mocker.patch('app.service_api_client.update_service_with_properties', return_value=service_one)
+    service_one['permissions'] = initial_permissions
 
-    response = logged_in_platform_admin_client.get(
-        url_for('main.service_switch_can_send_letters', service_id=service_one['id'])
+    page = client_request.get(
+        'main.service_set_letters',
+        service_id=service_one['id'],
     )
 
-    assert response.status_code == 302
-    assert response.location == url_for('main.service_settings', service_id=service_one['id'], _external=True)
-    assert 'letter' in mocked_fn.call_args[0][1]['permissions']
+    assert page.select_one('input[checked]')['value'] == expected_initial_value
+    assert len(page.select('input[checked]')) == 1
+
+    client_request.post(
+        'main.service_set_letters',
+        service_id=service_one['id'],
+        _data={'enabled': posted_value},
+        _expected_redirect=url_for(
+            'main.service_settings',
+            service_id=service_one['id'],
+            _external=True
+        )
+    )
+
+    assert set(mocked_fn.call_args[0][1]['permissions']) == set(expected_updated_permissions)
     assert mocked_fn.call_args[0][0] == service_one['id']
 
 
-def test_switch_service_disable_letters(
-    logged_in_platform_admin_client,
-    service_one,
-    mocker,
-):
-    service_one['permissions'] = ['letter']
-    mocked_fn = mocker.patch('app.service_api_client.update_service_with_properties', return_value=service_one)
-
-    response = logged_in_platform_admin_client.get(
-        url_for('main.service_switch_can_send_letters', service_id=service_one['id'])
-    )
-
-    assert response.status_code == 302
-    assert response.location == url_for('main.service_settings', service_id=service_one['id'], _external=True)
-    assert mocked_fn.call_args == call(service_one['id'], {"permissions": []})
-
-
 @pytest.mark.parametrize('permissions, expected_checked', [
     (['international_sms'], 'on'),
     ([''], 'off'),
@@ -1802,21 +1812,6 @@ def test_cant_resume_active_service(
 
 
 @pytest.mark.parametrize('endpoint, permissions, expected_p', [
-    (
-        'main.service_set_letters',
-        [],
-        (
-            'Using GOV.UK Notify to send letters is an invitation‑only '
-            'feature.'
-        )
-    ),
-    (
-        'main.service_set_letters',
-        ['letter'],
-        (
-            'Your service can send letters.'
-        )
-    ),
     (
         'main.service_set_inbound_sms',
         ['sms'],