FIX: Correctly sanitize negative integers in site settings (#24170)

As part of #23816, which sought to strip out thousand separators, we also accidentally strip out signs. This is making it impossible to disable some settings which require a -1 to disable. Instead of stripping non-digits, strip anything that isn't a sign or a digit.

spec/requests/admin/site_settings_controller_spec.rb

@@ -276,6 +276,16 @@ RSpec.describe Admin::SiteSettingsController do
         expect(SiteSetting.suggested_topics).to eq(1000)
       end
 
+      it "sanitizes negative integer values correctly" do
+        put "/admin/site_settings/pending_users_reminder_delay_minutes.json",
+            params: {
+              pending_users_reminder_delay_minutes: "-1",
+            }
+
+        expect(response.status).to eq(200)
+        expect(SiteSetting.pending_users_reminder_delay_minutes).to eq(-1)
+      end
+
       context "with default user options" do
         let!(:user1) { Fabricate(:user) }
         let!(:user2) { Fabricate(:user) }