Update PostSerializer and spec

[Refs #72500842]
Updated serializer and posts_controller_spec:
- added association to the serializer
- extracted post_response method in spec
Update PostsController

[Refs #72500842]
Added createComment method that adds comment to the post.
Update User model

[Refs #72500842]
Now following_posts method returns only posts (not comments)
Update stream templates

[Refs #72500842]
Added comments, form for submitting comments.