allow user to follow stories and discussions

[madhums]

Following #38, we'd want a user to follow stories and discussions.

• When a user follows a story, he gets notified of any new story updates and discussions.
• When a user follows a discussion, he gets notified of posts.

We can add a polymorphic association like so:

# app/models/user_subscription.rb
class UserSubscription < ApplicationRecord
  belongs_to :user
  belongs_to :subscribable, polymorphic: true
end

# app/models/user.rb
class User < ApplicationRecord
  has_many :user_subscriptions
end

# app/models/story.rb
class Story < ApplicationRecord
  has_many :discussions
  has_many :user_subscriptions, as: :subscribable
end

# app/models/discussion.rb
class Discussion < ApplicationRecord
  belongs_to :story
  has_many :posts
  has_many :user_subscriptions, as: :subscribable
end