Accept phone numbers in any valid format

This uses the `format_phone_number` method from utils to output phone numbers
in a consistent format. It is added to the schemas, so will be applied before
the API tries to do anything with a provided phone number.

So now the API will accept any of the following:
- 07123456789
- 07123 456789
- 07123-456-789
- 00447123456789
- 00 44 7123456789
- +447123456789
- +44 7123 456 789
- +44 (0)7123 456 789

…but the API will always hand off phone numbers to 3rd party APIs in the format
- +447123456789

The test for this is slightly convoluted, because template IDs are still
database IDs, and can’t consistently be mocked, therefore we have to ignore that
part of the call to `encrypt()`.

tests/app/notifications/test_rest.py

@@ -504,7 +504,7 @@ def test_should_allow_valid_sms_notification(notify_api, sample_template, mocker
             mocker.patch('app.encryption.encrypt', return_value="something_encrypted")
 
             data = {
-                'to': '+447700900855',
+                'to': '07700 900 855',
                 'template': sample_template.id
             }
 
@@ -521,6 +521,7 @@ def test_should_allow_valid_sms_notification(notify_api, sample_template, mocker
                 headers=[('Content-Type', 'application/json'), auth_header])
 
             notification_id = json.loads(response.data)['notification_id']
+            assert app.encryption.encrypt.call_args[0][0]['to'] == '+447700900855'
             app.celery.tasks.send_sms.apply_async.assert_called_once_with(
                 (str(sample_template.service_id),
                  notification_id,