Move self destruct check to config_for and add constant for verifier string (#32943)

app/helpers/self_destruct_helper.rb

@@ -1,9 +1,11 @@
 # frozen_string_literal: true
 
 module SelfDestructHelper
+  VERIFY_PURPOSE = 'self-destruct'
+
   def self.self_destruct?
-    value = ENV.fetch('SELF_DESTRUCT', nil)
-    value.present? && Rails.application.message_verifier('self-destruct').verify(value) == ENV['LOCAL_DOMAIN']
+    value = Rails.configuration.x.mastodon.self_destruct_value
+    value.present? && Rails.application.message_verifier(VERIFY_PURPOSE).verify(value) == ENV['LOCAL_DOMAIN']
   rescue ActiveSupport::MessageVerifier::InvalidSignature
     false
   end