Add New Comment on your post notifications

This notification is sent only to the post owner, unless this person turned off the notifications.
A simple first step into the notifications by mail world :)

The mail contains a link to user profile
The link to the user profile is required to give an easy access to
notifications disabling.

Also having a preview for the notify_post_owner method

We can `Comment.first` because it is part of the db:seeds method. So
there should, in development, always be one.

app/mailers/user_mailer.rb

@@ -0,0 +1,10 @@
+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}")
+  end
+end