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>

spec/controllers/auth/registrations_controller_spec.rb

@@ -82,6 +82,10 @@ RSpec.describe Auth::RegistrationsController, type: :controller do
   describe 'POST #create' do
     let(:accept_language) { Rails.application.config.i18n.available_locales.sample.to_s }
 
+    before do
+      session[:registration_form_time] = 5.seconds.ago
+    end
+
     around do |example|
       current_locale = I18n.locale
       example.run