Validate recipients in send a one-off message

It would be annoying to get all the way to the end of the flow and get
told that the phone number or email address you entered isn’t valid.

So this commit reuses the existing WTForms objects that we have to do
some extra validation on the first step in the send one-off message
flow. It also accounts for international phone numbers, if the service
is allowed to send them.

It doesn’t reject other people’s phone numbers if your service is
restricted, because I think it’s better to let users play with the
feature – it’s good for learning.

tests/app/main/test_placeholder_form.py

@@ -1,3 +1,4 @@
+import pytest
 from app.main.forms import get_placeholder_form_instance
 from wtforms import Label
 
@@ -16,3 +17,46 @@ def test_form_class_not_mutated(app_):
 
         assert str(form1.placeholder_value.label) == '<label for="placeholder_value">name</label>'
         assert str(form2.placeholder_value.label) == '<label for="placeholder_value">city</label>'
+
+
+@pytest.mark.parametrize('service_can_send_international_sms, placeholder_name, value, expected_error', [
+
+    (False, 'email address', '', 'Can’t be empty'),
+    (False, 'email address', '12345', 'Enter a valid email address'),
+    (False, 'email address', 'test@example.com', None),
+    (False, 'email address', 'test@example.gov.uk', None),
+
+    (False, 'phone number', '', 'Can’t be empty'),
+    (False, 'phone number', '+1-2345-678890', 'Not a UK mobile number'),
+    (False, 'phone number', '07900900123', None),
+    (False, 'phone number', '+44(0)7900 900-123', None),
+
+    (True, 'phone number', '+123', 'Not enough digits'),
+    (True, 'phone number', '+44(0)7900 900-123', None),
+    (True, 'phone number', '+1-2345-678890', None),
+
+    (False, 'anything else', '', 'Can’t be empty'),
+
+])
+def test_validates_recipients(
+    app_,
+    placeholder_name,
+    value,
+    service_can_send_international_sms,
+    expected_error,
+):
+    with app_.test_request_context(
+        method='POST',
+        data={'placeholder_value': value}
+    ):
+        form = get_placeholder_form_instance(
+            placeholder_name,
+            {},
+            allow_international_phone_numbers=service_can_send_international_sms,
+        )
+
+        if expected_error:
+            assert not form.validate_on_submit()
+            assert form.placeholder_value.errors[0] == expected_error
+        else:
+            assert form.validate_on_submit()