Migrate admin email checkboxes

Migrate the checkboxes to use
gitlab ui styles.

Changelog: changed

app/views/admin/application_settings/_email.html.haml

-= form_for @application_setting, url: preferences_admin_application_settings_path(anchor: 'js-email-settings'), html: { class: 'fieldset-form' } do |f|
+= gitlab_ui_form_for @application_setting, url: preferences_admin_application_settings_path(anchor: 'js-email-settings'), html: { class: 'fieldset-form' } do |f|
   = form_errors(@application_setting)
 
   %fieldset
     .form-group
-      .form-check
-        = f.check_box :email_author_in_body, class: 'form-check-input'
-        = f.label :email_author_in_body, class: 'form-check-label' do
-          = _('Include author name in notification email body')
-        .form-text.text-muted
-          = _("Include the name of the author of the issue, merge request or comment in the email body. By default, GitLab overrides the email sender's name. Some email servers don't support that option.")
+      = f.gitlab_ui_checkbox_component :email_author_in_body, _('Include author name in notification email body'), help_text: _("Include the name of the author of the issue, merge request or comment in the email body. By default, GitLab overrides the email sender's name. Some email servers don't support that option.")
     .form-group
-      .form-check
-        = f.check_box :html_emails_enabled, class: 'form-check-input'
-        = f.label :html_emails_enabled, class: 'form-check-label' do
-          = _('Enable multipart emails')
-        .form-text.text-muted
-          = _('Send email in multipart format (HTML and plain text). Uncheck to send email messages in plain text only.')
+      = f.gitlab_ui_checkbox_component :html_emails_enabled, _('Enable multipart emails'), help_text: _('Send email in multipart format (HTML and plain text). Uncheck to send email messages in plain text only.')
     .form-group
       = f.label :commit_email_hostname, _('Custom hostname (for private commit emails)'), class: 'label-bold'
       = f.text_field :commit_email_hostname, class: 'form-control gl-form-input'
@@ -26,19 +16,9 @@
     = render_if_exists 'admin/application_settings/email_additional_text_setting', form: f
 
     .form-group
-      .form-check
-        = f.check_box :in_product_marketing_emails_enabled, class: 'form-check-input'
-        = f.label :in_product_marketing_emails_enabled, class: 'form-check-label' do
-          = _('Enable in-product marketing emails')
-        .form-text.text-muted
-          = _('Send emails to help guide new users through the onboarding process.')
+      = f.gitlab_ui_checkbox_component :in_product_marketing_emails_enabled, _('Enable in-product marketing emails'), help_text: _('Send emails to help guide new users through the onboarding process.')
 
     .form-group
-      .form-check
-        = f.check_box :user_deactivation_emails_enabled, class: 'form-check-input'
-        = f.label :user_deactivation_emails_enabled, class: 'form-check-label' do
-          = _('Enable user deactivation emails')
-        .form-text.text-muted
-          = _('Send emails to users upon account deactivation.')
+      = f.gitlab_ui_checkbox_component :user_deactivation_emails_enabled, _('Enable user deactivation emails'), help_text: _('Send emails to users upon account deactivation.')
 
   = f.submit _('Save changes'), class: "gl-button btn btn-confirm", data: { qa_selector: 'save_changes_button' }