Re-import user's pubkey on access

Sometimes, the pubkey might not be imported in the local keychain
(anymore), but at this point in the code it had been successfully
imported at least once before. So we just (re-)import every time for it
to never fail.

app/models/user.rb

@@ -174,7 +174,8 @@ class User < ApplicationRecord
 
   def gnupg_key
     return nil unless pgp_pubkey.present?
-    @gnupg_key ||= GPGME::Key.get(pgp_fpr)
+    GPGME::Key.import(pgp_pubkey)
+    GPGME::Key.get(pgp_fpr)
   end
 
   def pgp_pubkey_contains_user_address?