Generate migration for comments

[Refs #72500842]

Generated migrations for the comments (parent_id),
updated schema.

Add self-reference to the Post model

[Refs #72500842]

Update Post model

[Refs #72500842]
Added scopes to the model.

Update User model

[Refs #72500842]

Added delegate to the User model (so we can get all of user’s comments
by current_user.comments

Update PostsController

[Refs #72500842]
Permit parent_id to be passed to the model.

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.
Merge branch 'ft/user-post-comments-72500842' into development

Conflicts:
	app/assets/javascripts/app/controllers/PostsCtrl.js.coffee