Fix default configuration for Devise.secret_key

This is the default configuration Devise uses. What we had previously
sets the `Devise.secret_key` to a random value at application boot that
is not known across different instances of the same application.

Author: senemsoy

lib/generators/solidus/auth/install/templates/config/initializers/devise.rb

@@ -1,4 +1,4 @@
 # frozen_string_literal: true
 
-Devise.secret_key = SecureRandom.hex(50).inspect
+Devise.secret_key = '<%= SecureRandom.hex(64) %>'
 Devise.email_regexp = Spree::Config[:default_email_regexp]