Let users switch channels on and off by themselves

We let people do this for letters already. We should let them do it for
emails and texts too, rather than have to email us.

tests/app/main/views/test_service_settings.py

@@ -2542,19 +2542,91 @@ def test_should_set_sms_allowance(
     )
 
 
+def test_old_set_letters_page_redirects(
+    client_request,
+):
+    client_request.get(
+        'main.service_set_letters',
+        service_id=SERVICE_ONE_ID,
+        _expected_status=302,
+        _expected_redirect=url_for(
+            'main.service_set_channel',
+            service_id=SERVICE_ONE_ID,
+            channel='letter',
+            _external=True,
+        )
+    )
+
+
+def test_unknown_channel_404s(
+    client_request,
+):
+    client_request.get(
+        'main.service_set_channel',
+        service_id=SERVICE_ONE_ID,
+        channel='message-in-a-bottle',
+        _expected_status=404,
+    )
+
+
 @pytest.mark.parametrize((
+    'channel,'
+    'expected_first_para,'
+    'expected_legend,'
+    'initial_permissions,'
     'expected_initial_value,'
     'posted_value,'
-    'initial_permissions,'
     'expected_updated_permissions'
 ), [
-    ('off', 'on', ['email', 'sms'], ['email', 'sms', 'letter']),
-    ('on', 'off', ['email', 'sms', 'letter'], ['email', 'sms']),
+    (
+        'letter',
+        'It costs between 30p and 76p to send a letter using Notify.',
+        'Send letters',
+        ['email', 'sms'],
+        'off', 'on',
+        ['email', 'sms', 'letter'],
+    ),
+    (
+        'letter',
+        'It costs between 30p and 76p to send a letter using Notify.',
+        'Send letters',
+        ['email', 'sms', 'letter'],
+        'on', 'off',
+        ['email', 'sms'],
+    ),
+    (
+        'sms',
+        'You have a free allowance of 250,000 text messages each financial year.',
+        'Send text messages',
+        [],
+        'off', 'on',
+        ['sms'],
+    ),
+    (
+        'email',
+        'It’s free to send emails through GOV.UK Notify.',
+        'Send emails',
+        [],
+        'off', 'on',
+        ['email'],
+    ),
+    (
+        'email',
+        'It’s free to send emails through GOV.UK Notify.',
+        'Send emails',
+        ['email', 'sms', 'letter'],
+        'on', 'on',
+        ['email', 'sms', 'letter'],
+    ),
 ])
 def test_switch_service_enable_letters(
     client_request,
     service_one,
     mocker,
+    mock_get_free_sms_fragment_limit,
+    channel,
+    expected_first_para,
+    expected_legend,
     expected_initial_value,
     posted_value,
     initial_permissions,
@@ -2564,16 +2636,21 @@ def test_switch_service_enable_letters(
     service_one['permissions'] = initial_permissions
 
     page = client_request.get(
-        'main.service_set_letters',
+        'main.service_set_channel',
         service_id=service_one['id'],
+        channel=channel,
     )
 
+    assert normalize_spaces(page.select_one('main p').text) == expected_first_para
+    assert normalize_spaces(page.select_one('legend').text) == expected_legend
+
     assert page.select_one('input[checked]')['value'] == expected_initial_value
     assert len(page.select('input[checked]')) == 1
 
     client_request.post(
-        'main.service_set_letters',
+        'main.service_set_channel',
         service_id=service_one['id'],
+        channel=channel,
         _data={'enabled': posted_value},
         _expected_redirect=url_for(
             'main.service_settings',