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/controllers/follows_controller.rb

@@ -0,0 +1,53 @@
+class FollowsController < ApplicationController
+  before_action :authenticate_user!, only: [:create, :destroy]
+
+  def index
+    unless user_signed_in?
+      render json: { }
+      return
+    end
+
+    follow = Follow.find_by(follow_params)
+    render json: follow
+  end
+
+  def create
+    follow = Follow.new(follow_params)
+
+    if follow.save
+      render json: {
+        id: follow.id
+      }, status: :created
+    else
+      render json: {
+        error: I18n.t('errors.follows.create', message: follow.errors.full_messages)
+      }, status: :unprocessable_entity
+    end
+  end
+
+  def destroy
+    follow = Follow.find_by(follow_params)
+    id = follow.id
+    
+    return if follow.nil?
+
+    if follow.destroy
+      render json: {
+        id: id,
+      }, status: :accepted
+    else
+      render json: {
+        error: I18n.t('errors.follow.destroy', message: follow.errors.full_messages)
+      }, status: :unprocessable_entity
+    end
+  end
+
+  private
+
+    def follow_params
+      {
+        post_id: params[:post_id],
+        user_id: current_user.id,
+      }
+    end
+end