Add honeypot fields and minimum fill-out time for sign-up form (#15276)

* Add honeypot fields to limit non-specialized spam

Add two honeypot fields: a fake website input and a fake password confirmation
one. The label/placeholder/aria-label tells not to fill them, and they are
hidden in CSS, so legitimate users should not fall into these.

This should cut down on some non-Mastodon-specific spambots.

* Require a 3 seconds delay before submitting the registration form

* Fix tests

* Move registration form time check to model validation

* Give people a chance to clear the honeypot fields

* Refactor honeypot translation strings

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/packs/public.js

@@ -280,6 +280,17 @@ function main() {
       target.style.display = 'block';
     }
   });
+
+  // Empty the honeypot fields in JS in case something like an extension
+  // automatically filled them.
+  delegate(document, '#registration_new_user,#new_user', 'submit', () => {
+    ['user_website', 'user_confirm_password', 'registration_user_website', 'registration_user_confirm_password'].forEach(id => {
+      const field = document.getElementById(id);
+      if (field) {
+        field.value = '';
+      }
+    });
+  });
 }
 
 loadPolyfills()