feat(messages): render markdown in message body

uses redcarpet gem to render markdown to html. Also a html email template
is added.

plugins/messages/app/models/message.rb

@@ -132,9 +132,14 @@ class Message < ApplicationRecord
     user.role_admin?
   end
 
+  def render_markdown
+    markdown = Redcarpet::Markdown.new(Redcarpet::Render::HTML, autolink: true, tables: true, strikethrough: true, footnotes: true)
+    return markdown.render(body)
+  end
+
   private
 
   def create_salt
     self.salt = [Array.new(6) { rand(256).chr }.join].pack("m").chomp
   end
-end
+end