Allow admin app to specify domain for password reset

This follows the pattern for invite emails where the admin app tells the
API which domain to use when generating the link.

This will starting working once this admin change is merged:
- [ ] https://github.com/alphagov/notifications-admin/pull/4150/files

It won’t break anything if it’s merged before the admin change.

tests/app/user/test_rest.py

@@ -667,6 +667,39 @@ def test_send_user_reset_password_should_send_reset_password_link(client,
     assert notification.reply_to_text == notify_service.get_default_reply_to_email_address()
 
 
+@pytest.mark.parametrize('data, expected_url', (
+    ({
+        'email': 'notify@digital.cabinet-office.gov.uk',
+    }, (
+        'http://localhost:6012/new-password/'
+    )),
+    ({
+        'email': 'notify@digital.cabinet-office.gov.uk',
+        'admin_base_url': 'https://different.example.com',
+    }, (
+        'https://different.example.com/new-password/'
+    )),
+))
+@freeze_time("2016-01-01 11:09:00.061258")
+def test_send_user_reset_password_should_use_provided_base_url(
+    admin_request,
+    sample_user,
+    password_reset_email_template,
+    mocker,
+    data,
+    expected_url,
+):
+    mocker.patch('app.celery.provider_tasks.deliver_email.apply_async')
+
+    admin_request.post(
+        'user.send_user_reset_password',
+        _data=data,
+        _expected_status=204,
+    )
+
+    assert Notification.query.first().personalisation['url'].startswith(expected_url)
+
+
 @freeze_time("2016-01-01 11:09:00.061258")
 def test_send_user_reset_password_reset_password_link_contains_redirect_link_if_present_in_request(
     client, sample_user, mocker, password_reset_email_template