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'
@@ -75,6 +76,7 @@ gem 'foodsoft_polls', path: 'plugins/polls'
 # gem 'foodsoft_printer', path: 'plugins/printer'
 # gem 'foodsoft_uservoice', path: 'plugins/uservoice'
 
+
 group :development do
   gem 'sqlite3', '~> 1.3.6'
   gem 'mailcatcher'
@@ -126,3 +128,5 @@ group :test do
   gem 'rswag-specs'
   gem 'hashie', '~> 3.4.6', require: false # https://github.com/westfieldlabs/apivore/issues/114
 end
+
+gem "redcarpet", "~> 3.6"