Post follow and updates notifications V1 (#111)

* It is now possible to follow a post in order to receive updates about it
* Notifications are now sent when updates are published
* Post status changes are now tracked
* Update sidebar now shows the post status history
* Mark a comment as a post update using the comment form
* ... more ...

app/models/post.rb

@@ -2,8 +2,12 @@ class Post < ApplicationRecord
   belongs_to :board
   belongs_to :user
   belongs_to :post_status, optional: true
+
   has_many :likes, dependent: :destroy
+  has_many :follows, dependent: :destroy
+  has_many :followers, through: :follows, source: :user
   has_many :comments, dependent: :destroy
+  has_many :post_status_changes, dependent: :destroy
 
   validates :title, presence: true, length: { in: 4..64 }