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.

app/user/rest.py

@@ -499,10 +499,9 @@ def find_users_by_email():
 
 @user_blueprint.route('/reset-password', methods=['POST'])
 def send_user_reset_password():
-    email, errors = email_data_request_schema.load(request.get_json())
-
+    request_json = request.get_json()
+    email, errors = email_data_request_schema.load(request_json)
     user_to_send_to = get_user_by_email(email['email'])
-
     template = dao_get_template_by_id(current_app.config['PASSWORD_RESET_TEMPLATE_ID'])
     service = Service.query.get(current_app.config['NOTIFY_SERVICE_ID'])
     saved_notification = persist_notification(
@@ -514,7 +513,8 @@ def send_user_reset_password():
             'user_name': user_to_send_to.name,
             'url': _create_reset_password_url(
                 user_to_send_to.email_address,
-                next_redirect=request.get_json().get('next')
+                base_url=request_json.get('admin_base_url'),
+                next_redirect=request_json.get('next')
             )
         },
         notification_type=template.template_type,
@@ -547,10 +547,10 @@ def get_organisations_and_services_for_user(user_id):
     return jsonify(data)
 
 
-def _create_reset_password_url(email, next_redirect):
+def _create_reset_password_url(email, next_redirect, base_url=None):
     data = json.dumps({'email': email, 'created_at': str(datetime.utcnow())})
     static_url_part = '/new-password/'
-    full_url = url_with_token(data, static_url_part, current_app.config)
+    full_url = url_with_token(data, static_url_part, current_app.config, base_url=base_url)
     if next_redirect:
         full_url += '?{}'.format(urlencode({'next': next_redirect}))
     return full_url