Let users register with int’national phone numbers

Right now Notify restricts you to registering with a UK mobile number.
This is because when we built the user registration stuff we couldn’t
send to international mobiles.

However we can send to international mobile numbers, and it’s totally
reasonable to expect employees of the UK government to be working
abroad, and have a foreign mobile phone – we’ve heard from one such
user.

So this commit:
- changes all places where users enter their own phone number to use
  the validation function which allows international phone numbers
- renames the `mobile_number` validation to `uk_mobile_number` to make
  it explicit, and force it to break the tests if there’s somewhere it’s
  being used that I haven’t thought of

tests/app/main/views/test_code_not_received.py

@@ -71,12 +71,17 @@ def test_should_render_correct_resend_template_for_pending_user(
     assert page.find('form').input['value'] == api_user_pending.mobile_number
 
 
+@pytest.mark.parametrize('phone_number_to_register_with', [
+    '+447700900460',
+    '+1800-555-555',
+])
 def test_should_resend_verify_code_and_update_mobile_for_pending_user(
     client,
     mocker,
     api_user_pending,
     mock_update_user,
     mock_send_verify_code,
+    phone_number_to_register_with,
 ):
 
     mocker.patch('app.user_api_client.get_user_by_email', return_value=api_user_pending)
@@ -86,12 +91,12 @@ def test_should_resend_verify_code_and_update_mobile_for_pending_user(
             'id': api_user_pending.id,
             'email': api_user_pending.email_address}
     response = client.post(url_for('main.check_and_resend_text_code'),
-                           data={'mobile_number': '+447700900460'})
+                           data={'mobile_number': phone_number_to_register_with})
     assert response.status_code == 302
     assert response.location == url_for('main.verify', _external=True)
 
     mock_update_user.assert_called_once_with(api_user_pending)
-    mock_send_verify_code.assert_called_once_with(api_user_pending.id, 'sms', to='+447700900460')
+    mock_send_verify_code.assert_called_once_with(api_user_pending.id, 'sms', to=phone_number_to_register_with)
 
 
 def test_check_and_redirect_to_two_factor_if_user_active(