Source vapid subject from vapid_subject env var

The VAPID JWT subject defaulted to `mailto:support@fizzy.do`. It could already be overridden by setting config.x.vapid.subject (the merge in vapid_identification took care of that), but for self-hosted deployments this wasn't obvious and was easily missed, so they'd silently ship the fizzy.do address.

This change keeps the current default but lets the subject be overridden via an env var.

Author: ryanharkins

config/initializers/vapid.rb

@@ -1,4 +1,5 @@
 Rails.application.configure do
   config.x.vapid.private_key = ENV["VAPID_PRIVATE_KEY"]
   config.x.vapid.public_key = ENV["VAPID_PUBLIC_KEY"]
+  config.x.vapid.subject = ENV.fetch("VAPID_SUBJECT", "mailto:support@fizzy.do")
 end

lib/web_push/notification.rb

@@ -15,8 +15,7 @@ def deliver(connection: nil)
 
   private
     def vapid_identification
-      { subject: "mailto:support@fizzy.do" }.merge \
-        Rails.configuration.x.vapid.symbolize_keys
+      Rails.configuration.x.vapid.symbolize_keys
     end
 
     def encoded_message