Fix posts from your own webhook not showing as unread until refresh (#5703)

webapp/utils/post_utils.jsx

@@ -12,6 +12,10 @@ export function isSystemMessage(post) {
     return post.type && (post.type.lastIndexOf(Constants.SYSTEM_MESSAGE_PREFIX) === 0);
 }
 
+export function isFromWebhook(post) {
+    return post.props && post.props.from_webhook === 'true';
+}
+
 export function isPostOwner(post) {
     return UserStore.getCurrentId() === post.user_id;
 }