feat(messages): render markdown in message body

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

app/helpers/application_helper.rb

@@ -263,4 +263,26 @@ module ApplicationHelper
       stylesheet_link_tag foodcoop_css_path, media: 'all'
     end
   end
+
+  # renders html from markdown input
+  def markdown(text)
+    options = {
+        filter_html: true,
+        hard_wrap: true,
+        link_attributes: { rel: 'nofollow', target: "_blank" },
+        fenced_code_blocks: true
+      }
+      extensions = {
+        autolink: true,
+        superscript: true,
+        disable_indented_code_blocks: true,
+        tables: true,
+        strikethrough: true,
+        footnotes: true
+    }
+
+    renderer = ::Redcarpet::Render::HTML.new(options)
+    markdown = ::Redcarpet::Markdown.new(renderer, extensions)
+    markdown.render(text).html_safe
+  end
 end