Set autocomplete attr for email field on signup page (#19833)

The email address will be used as the "username" for sign-in purposes, so it's the value that should be stored in password managers. We can inform the password manager of this by setting `autocomplete="email"`. Without this hint, password managers may instead store the `username` field, which isn't valid for sign-in (this happens with iCloud Keychain in Safari, for instance).

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

@@ -19,7 +19,7 @@
     = f.simple_fields_for :account do |ff|
       = ff.input :display_name, wrapper: :with_label, label: false, required: false, input_html: { 'aria-label' => t('simple_form.labels.defaults.display_name'), :autocomplete => 'off', placeholder: t('simple_form.labels.defaults.display_name') }
       = ff.input :username, wrapper: :with_label, label: false, required: true, input_html: { 'aria-label' => t('simple_form.labels.defaults.username'), :autocomplete => 'off', placeholder: t('simple_form.labels.defaults.username'), pattern: '[a-zA-Z0-9_]+', maxlength: 30 }, append: "@#{site_hostname}", hint: false
-    = f.input :email, placeholder: t('simple_form.labels.defaults.email'), required: true, input_html: { 'aria-label' => t('simple_form.labels.defaults.email'), :autocomplete => 'off' }, hint: false
+    = f.input :email, placeholder: t('simple_form.labels.defaults.email'), required: true, input_html: { 'aria-label' => t('simple_form.labels.defaults.email'), :autocomplete => 'username' }, hint: false
     = f.input :password, placeholder: t('simple_form.labels.defaults.password'), required: true, input_html: { 'aria-label' => t('simple_form.labels.defaults.password'), :autocomplete => 'new-password', :minlength => User.password_length.first, :maxlength => User.password_length.last }, hint: false
     = f.input :password_confirmation, placeholder: t('simple_form.labels.defaults.confirm_password'), required: true, input_html: { 'aria-label' => t('simple_form.labels.defaults.confirm_password'), :autocomplete => 'new-password' }, hint: false
     = f.input :confirm_password, as: :string, placeholder: t('simple_form.labels.defaults.honeypot', label: t('simple_form.labels.defaults.password')), required: false, input_html: { 'aria-label' => t('simple_form.labels.defaults.honeypot', label: t('simple_form.labels.defaults.password')), :autocomplete => 'off' }, hint: false