FIX: Do not interpret backreferences in email content

This changes from providing a string literal for the #sub replacement, to providing a block.
Because the block is provided the match object, it is presumed to have already performed all necessary backreferences.
This avoids any replacement of backreferences in the message body.

app/helpers/email_helper.rb

@@ -27,7 +27,7 @@ module EmailHelper
 
   def email_html_template
     EmailStyle.new.html
-      .sub('%{email_content}', capture { yield })
+      .sub('%{email_content}') { capture { yield } }
       .gsub('%{html_lang}', html_lang)
       .html_safe
   end