dmails, emails: refactor to use Rails signed_id.

Refactor email verification links and Dmail share links to use the new
Rails signed_id mechanism, rather than our own handrolled mechanism.

For Dmail share links, we have to override some Rails internal methods
so that our old links still work. For email verification links, this
will invalidate existing links, but this isn't a huge deal since these
links are short-lived anyway.

https://api.rubyonrails.org/classes/ActiveRecord/SignedId.html
https://api.rubyonrails.org/classes/ActiveRecord/SignedId/ClassMethods.html

app/models/email_address.rb

@@ -69,17 +69,7 @@ class EmailAddress < ApplicationRecord
     end
   end
 
-  concerning :VerificationMethods do
-    def verifier
-      @verifier ||= Danbooru::MessageVerifier.new(:email_verification_key)
-    end
-
-    def verification_key
-      verifier.generate(id)
-    end
-
-    def valid_key?(key)
-      id == verifier.verified(key)
-    end
+  def verification_key
+    signed_id(purpose: "verify")
   end
 end