Add HTML emails for Service Desk thank-you and new note

Thanks @smcgivern for the layout tips

app/mailers/emails/ee/service_desk.rb

 module Emails
   module EE
     module ServiceDesk
+      extend ActiveSupport::Concern
+
+      included do
+        layout 'service_desk', only: [:service_desk_thank_you_email, :service_desk_new_note_email]
+      end
+
       def service_desk_thank_you_email(issue_id)
         setup_service_desk_mail(issue_id)
 

app/views/layouts/service_desk.html.haml

+%html{ lang: "en" }
+  %head
+    %meta{ content: "text/html; charset=utf-8", "http-equiv" => "Content-Type" }
+    %title
+      GitLab
+    = stylesheet_link_tag 'notify'
+    = yield :head
+  %body
+    .content
+      = yield
+    .footer{ style: "margin-top: 10px;" }
+    %p
+      —
+      %br
+      = link_to "Unsubscribe", @sent_notification_url

app/views/notify/service_desk_new_note_email.html.haml

+- if current_application_settings.email_author_in_body
+  %div
+    #{link_to @note.author_name, user_url(@note.author)} wrote:
+%div
+  = markdown(@note.note, pipeline: :email, author: @note.author)

app/views/notify/service_desk_thank_you_email.html.haml

+%p
+  Thank you for your support request! We are tracking your request as ticket ##{@issue.iid}, and will respond as soon as we can.