votes: make private favorites and upvotes a Gold-only option.

Make private favorites and upvotes a Gold-only account option.

Existing Members with private favorites enabled are allowed to keep it
enabled, as long as they don't disable it. If they disable it, then they
can't re-enable it again without upgrading to Gold first.

This is a Gold-only option to prevent uploaders from creating multiple
accounts to upvote their own posts. If private upvotes were allowed for
Members, then it would be too easy to use fake accounts and private
upvotes to upvote your own posts.

test/functional/users_controller_test.rb

@@ -420,6 +420,34 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
         assert_equal("xyz", @user.favorite_tags)
       end
 
+      context "for a Member-level user" do
+        should "allow disabling the private favorites option" do
+          @user = create(:user, enable_private_favorites: true)
+          put_auth user_path(@user), @user, params: { user: { enable_private_favorites: false }}
+
+          assert_redirected_to edit_user_path(@user)
+          assert_equal(false, @user.reload.enable_private_favorites)
+        end
+
+        should "not allow enabling the private favorites option" do
+          @user = create(:user, enable_private_favorites: false)
+          put_auth user_path(@user), @user, params: { user: { enable_private_favorites: true }}
+
+          assert_redirected_to edit_user_path(@user)
+          assert_equal(false, @user.reload.enable_private_favorites)
+        end
+      end
+
+      context "for a Gold-level user" do
+        should "allow enabling the private favorites option" do
+          @user = create(:gold_user, enable_private_favorites: false)
+          put_auth user_path(@user), @user, params: { user: { enable_private_favorites: true }}
+
+          assert_redirected_to edit_user_path(@user)
+          assert_equal(true, @user.reload.enable_private_favorites)
+        end
+      end
+
       context "changing the level" do
         should "not work" do
           @owner = create(:owner_user)