feat: use resize postmessage API to send size hints to parent (#100)

When the post list is resized, send a size hint to the parent window (LMS/Learning MFE) so it can resize the discussions MFE iframe dynamically.

.env.development

@@ -7,6 +7,7 @@ CSRF_TOKEN_API_PATH='/csrf/api/v1/token'
 ECOMMERCE_BASE_URL='http://localhost:18130'
 LANGUAGE_PREFERENCE_COOKIE_NAME='openedx-language-preference'
 LMS_BASE_URL='http://localhost:18000'
+LEARNING_BASE_URL='http://localhost:2000'
 LOGIN_URL='http://localhost:18000/login'
 LOGOUT_URL='http://localhost:18000/logout'
 LOGO_URL=https://edx-cdn.org/v3/default/logo.svg