replace user PUT with POSTs

the update_user fn was used in two places, for things that are handled
fine by update_user_attribute. Reduce complexity in the API by killing
the PUT, which is more dangerous (might silently overwrite things that
shouldn't be, like "last_logged_in_at" etc).

Had to change the code not received mobile number form, and the
activate user function.

tests/app/main/views/test_verify.py

@@ -30,7 +30,7 @@ def test_should_redirect_to_add_service_when_sms_code_is_correct(
     client,
     api_user_active,
     mocker,
-    mock_update_user,
+    mock_update_user_attribute,
     mock_check_verify_code,
     fake_uuid
 ):
@@ -60,14 +60,14 @@ def test_should_activate_user_after_verify(
     api_user_pending,
     mock_send_verify_code,
     mock_check_verify_code,
-    mock_update_user,
+    mock_activate_user,
 ):
     mocker.patch('app.user_api_client.get_user', return_value=api_user_pending)
     with client.session_transaction() as session:
         session['user_details'] = {'email_address': api_user_pending.email_address, 'id': api_user_pending.id}
     client.post(url_for('main.verify'),
                 data={'sms_code': '12345'})
-    assert mock_update_user.called
+    assert mock_activate_user.called
 
 
 def test_should_return_200_when_sms_code_is_wrong(