Set email autocomplete attribute for registrations

Based on https://github.com/mastodon/mastodon/pull/19833

app/views/auth/registrations/new.html.haml

@@ -17,7 +17,7 @@
       = ff.input :username, wrapper: :with_label, autofocus: true, label: t('simple_form.labels.defaults.username'), required: true, input_html: { 'aria-label' => t('simple_form.labels.defaults.username'), :autocomplete => 'off', pattern: '[a-zA-Z0-9_]+', maxlength: 30 }, append: "@#{site_hostname}", hint: t('simple_form.hints.defaults.username', domain: site_hostname)
 
   .fields-group
-    = f.input :email, wrapper: :with_label, label: t('simple_form.labels.defaults.email'), required: true, input_html: { 'aria-label' => t('simple_form.labels.defaults.email'), :autocomplete => 'off' }
+    = f.input :email, wrapper: :with_label, label: t('simple_form.labels.defaults.email'), required: true, input_html: { 'aria-label' => t('simple_form.labels.defaults.email'), :autocomplete => 'username' }
 
   .fields-group
     = f.input :password, wrapper: :with_label, label: t('simple_form.labels.defaults.password'), required: true, input_html: { 'aria-label' => t('simple_form.labels.defaults.password'), :autocomplete => 'off', :minlength => User.password_length.first, :maxlength => User.password_length.last }