Added a followed_posts method to User class.
This method executes a query which is three pronged in nature -:
1. First, a join is performed on the Posts (left) and followers (right) table, on the condition that followers.followed_id equals Post.user_id.
2. Next, a filter is applied where the follower_id in followers table must match the id of the target user, i.e., the user executing the query whose followed posts we wish to find.
3. Lastly, the result is sorted according to Post.timestamp.