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.

spec/mailers/user_mailer_spec.rb

@@ -0,0 +1,22 @@
+require "rails_helper"
+
+RSpec.describe UserMailer, type: :mailer do
+  describe "notify_comment" do
+    let(:user) { FactoryBot.create(:user, email: "notified@example.com", notifications_enabled: true) }
+    let(:post) { FactoryBot.create(:post, user: user)}
+    let(:comment) { FactoryBot.create(:comment, post: post) }
+    let(:mail) { UserMailer.notify_post_owner(comment: comment) }
+
+    it "renders the headers" do
+      expect(mail.subject).to eq("[#{ENV.fetch('APP_NAME')}] - New comment on #{post.title}")
+      expect(mail.to).to eq(["notified@example.com"])
+      expect(mail.from).to eq(["notifications@example.com"])
+    end
+
+    it "renders the body" do
+      expect(mail.body.encoded).to include("Hello, #{user.full_name}")
+      expect(mail.body.encoded).to include("There is a new comment by")
+      expect(mail.body.encoded).to include('Annoyed ? You can <a href="http://localhost:3000/users/edit">turn off notifications here</a>')
+    end
+  end
+end