users: require email verification for signups from proxies.

Require users who signup using proxies to verify their email addresses
before they can perform any edits. For verification purposes, the email
must be a nondisposable address from a whitelist of trusted email
providers.

config/danbooru_default_config.rb

@@ -519,6 +519,13 @@ module Danbooru
       nil
     end
 
+    # A list of email domains that are used for account verification purposes.
+    # If a user signs up from a proxy they will need to verify their account
+    # using an email address from one of the domains on this list.
+    def email_domain_verification_list
+      # ["gmail.com", "outlook.com", "yahoo.com"]
+    end
+
     # API key for Google Maps. Used for embedding maps on IP address lookup pages.
     # Generate at https://console.developers.google.com/apis/credentials
     def google_maps_api_key