Update user mailer from address

Send user mail from the blackhole address help reduce the amount of
email we receive to support inboxes.

Fixes #6404

app/mailers/application_mailer.rb

@@ -42,7 +42,7 @@ def mail_user(user, subject:, **opts)
     set_auto_generated_headers
 
     default_opts = {
-      from: contact_for_user(user),
+      from: blackhole_email,
       to: user.name_and_email,
       subject: subject
     }
@@ -82,8 +82,7 @@ def set_auto_generated_headers(_opts = {})
   #   from transitory bounce messages.
   def set_reply_to_headers(opts = {})
     default_opts = {
-      'Return-Path' => blackhole_email,
-      'Reply-To' => contact_for_user(user)
+      'Return-Path' => blackhole_email
     }
     default_opts.merge!(opts)
     headers(default_opts)