feat(messages): render markdown in message body

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

Gemfile

@@ -60,7 +60,6 @@ gem 'mime-types'
 gem 'midi-smtp-server'
 gem 'rswag-api'
 gem 'rswag-ui'
-gem 'maildown'
 
 # we use the git version of acts_as_versioned, and need to include it in this Gemfile
 gem 'acts_as_versioned', git: 'https://github.com/technoweenie/acts_as_versioned.git'

Gemfile.lock

@@ -283,10 +283,6 @@ GEM
       activerecord
       kaminari-core (= 1.2.2)
     kaminari-core (1.2.2)
-    kramdown (2.4.0)
-      rexml
-    kramdown-parser-gfm (1.1.0)
-      kramdown (~> 2.0)
     less (2.6.0)
       commonjs (~> 0.2.7)
     less-rails (5.0.0)
@@ -312,9 +308,6 @@ GEM
       skinny (>= 0.1.2)
       sqlite3-ruby
       thin
-    maildown (3.3.1)
-      actionmailer (>= 4.0.0)
-      kramdown-parser-gfm
     marcel (1.0.2)
     matrix (0.4.2)
     method_source (1.0.0)
@@ -643,7 +636,6 @@ DEPENDENCIES
   listen
   mail (~> 2.7.1)
   mailcatcher
-  maildown
   midi-smtp-server
   mime-types
   mysql2

plugins/messages/app/models/message.rb

@@ -142,5 +142,4 @@ class Message < ApplicationRecord
   def create_salt
     self.salt = [Array.new(6) { rand(256).chr }.join].pack("m").chomp
   end
-
 end