Allow admin to specify domain for email auth links

Similar to https://github.com/alphagov/notifications-api/pull/1515

This lets the admin app pass in a domain to use for email auth links,
so that when it’s running on a different URL users who try to sign in
will get an email auth link for the domain they sign in on, not the
default admin domain for the environment in which the API is running.

tests/app/user/test_rest_verify.py

@@ -343,12 +343,30 @@ def test_reset_failed_login_count_returns_404_when_user_does_not_exist(client):
     assert resp.status_code == 404
 
 
-def test_send_user_email_code(admin_request, mocker, sample_user, email_2fa_code_template):
+@pytest.mark.parametrize('data, expected_auth_url', (
+    (
+        {},
+        'http://localhost:6012/email-auth/.',
+    ),
+    (
+        {'to': None},
+        'http://localhost:6012/email-auth/.',
+    ),
+    (
+        {'to': None, 'email_auth_link_host': 'https://example.com'},
+        'https://example.com/email-auth/.',
+    ),
+))
+def test_send_user_email_code(
+    admin_request,
+    mocker,
+    sample_user,
+    email_2fa_code_template,
+    data,
+    expected_auth_url,
+):
     deliver_email = mocker.patch('app.celery.provider_tasks.deliver_email.apply_async')
 
-    data = {
-        'to': None
-    }
     admin_request.post(
         'user.send_user_2fa_code',
         code_type='email',
@@ -361,6 +379,7 @@ def test_send_user_email_code(admin_request, mocker, sample_user, email_2fa_code
     assert noti.to == sample_user.email_address
     assert str(noti.template_id) == current_app.config['EMAIL_2FA_TEMPLATE_ID']
     assert noti.personalisation['name'] == 'Test User'
+    assert noti.personalisation['url'].startswith(expected_auth_url)
     deliver_email.assert_called_once_with(
         [str(noti.id)],
         queue='notify-internal-tasks'