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

@@ -137,4 +137,4 @@ class Message < ApplicationRecord
   def create_salt
     self.salt = [Array.new(6) { rand(256).chr }.join].pack("m").chomp
   end
-end
+end