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_user_profile.py

@@ -1,3 +1,4 @@
+import pytest
 import uuid
 import json
 
@@ -117,16 +118,23 @@ def test_should_show_mobile_number_page(
     assert response.status_code == 200
 
 
+@pytest.mark.parametrize('phone_number_to_register_with', [
+    '+4407700900460',
+    '+1800-555-555',
+])
 def test_should_redirect_after_mobile_number_change(
     logged_in_client,
+    phone_number_to_register_with,
 ):
-    data = {'mobile_number': '07121231234'}
+    data = {'mobile_number': phone_number_to_register_with}
     response = logged_in_client.post(
         url_for('main.user_profile_mobile_number'),
         data=data)
     assert response.status_code == 302
     assert response.location == url_for(
         'main.user_profile_mobile_number_authenticate', _external=True)
+    with logged_in_client.session_transaction() as session:
+        assert session['new-mob'] == phone_number_to_register_with
 
 
 def test_should_show_authenticate_after_mobile_number_change(
@@ -172,12 +180,17 @@ def test_should_show_confirm_after_mobile_number_change(
     assert response.status_code == 200
 
 
+@pytest.mark.parametrize('phone_number_to_register_with', [
+    '+4407700900460',
+    '+1800-555-555',
+])
 def test_should_redirect_after_mobile_number_confirm(
     logged_in_client,
     mocker,
     mock_update_user_attribute,
     mock_check_verify_code,
-    fake_uuid
+    fake_uuid,
+    phone_number_to_register_with,
 ):
     user_before = create_user(fake_uuid)
     user_after = create_user(fake_uuid)
@@ -189,7 +202,7 @@ def test_should_redirect_after_mobile_number_confirm(
 
     with logged_in_client.session_transaction() as session:
         session['new-mob-password-confirmed'] = True
-        session['new-mob'] = '+441234123123'
+        session['new-mob'] = phone_number_to_register_with
         session['current_session_id'] = user_before.current_session_id
     data = {'sms_code': '12345'}
     response = logged_in_client.post(