feat(messages): render markdown in message body

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

Gemfile

@@ -60,6 +60,7 @@ 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

@@ -283,6 +283,10 @@ 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)
@@ -308,6 +312,9 @@ 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)
@@ -636,6 +643,7 @@ DEPENDENCIES
   listen
   mail (~> 2.7.1)
   mailcatcher
+  maildown
   midi-smtp-server
   mime-types
   mysql2

plugins/messages/app/models/message.rb

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