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/mailers/user_mailer.rb

@@ -1,10 +1,45 @@
 class UserMailer < ApplicationMailer
-  default from: "notifications@example.com"
-
   def notify_post_owner(comment:)
     @comment = comment
     @user = comment.post.user
 
-    mail(to: @user.email, subject: "[#{ENV.fetch('APP_NAME')}] - New comment on #{comment.post.title}")
+    mail(
+      to: @user.email,
+      subject: "[#{app_name}] New comment on #{comment.post.title}"
+    )
   end
+
+  def notify_comment_owner(comment:)
+    @comment = comment
+    @user = comment.parent.user
+
+    mail(
+      to: @user.email,
+      subject: "[#{app_name}] New reply on your comment from #{comment.post.title}"
+    )
+  end
+
+  def notify_followers_of_post_update(comment:)
+    @comment = comment
+
+    mail(
+      to: comment.post.followers.pluck(:email),
+      subject: "[#{app_name}] New update on #{comment.post.title}"
+    )
+  end
+
+  def notify_followers_of_post_status_change(post:)
+    @post = post
+
+    mail(
+      to: post.followers.pluck(:email),
+      subject: "[#{app_name}] Status change on post #{post.title}"
+    )
+  end
+
+  private
+
+    def app_name
+      ENV.fetch('APP_NAME')
+    end
 end