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.

test/functional/users_controller_test.rb

@@ -151,6 +151,24 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
         end
       end
 
+      should "mark users signing up from proxies as requiring verification" do
+        skip unless IpLookup.enabled?
+        self.remote_addr = "1.1.1.1"
+        post users_path, params: { user: { name: "xxx", password: "xxxxx1", password_confirmation: "xxxxx1" }}
+
+        assert_redirected_to User.last
+        assert_equal(true, User.last.requires_verification)
+      end
+
+      should "not mark users signing up from non-proxies as requiring verification" do
+        skip unless IpLookup.enabled?
+        self.remote_addr = "187.37.226.17"
+        post users_path, params: { user: { name: "xxx", password: "xxxxx1", password_confirmation: "xxxxx1" }}
+
+        assert_redirected_to User.last
+        assert_equal(false, User.last.requires_verification)
+      end
+
       context "with sockpuppet validation enabled" do
         setup do
           Danbooru.config.unstub(:enable_sock_puppet_validation?)