 Make a new post from 'author' to the 'target' user,
containing the given contents, IF the users are friends.
 
 Insert the new post at the *front* of the user's list of posts.
 
 'contents' is a pointer to heap-allocated memory - you do not need
 To allocate more memory to store the contents of the post.
 Return:
 - 0 on success
- 1 if users exist but are not friends
- 2 if either User pointer is NULL
