Replace generic checkbox with GitLab UI component

Updated the checkbox on the welcome screen.

Changelog: changed

app/views/devise/shared/_email_opted_in.html.haml

@@ -3,5 +3,4 @@
 .gl-mb-3.js-email-opt-in.hidden
   .gl-font-weight-bold.gl-mb-3
     = _('Email updates (optional)')
-  = f.check_box :email_opted_in
-  = f.label :email_opted_in, _("I'd like to receive updates about GitLab via email"), class: 'gl-font-weight-normal'
+  = f.gitlab_ui_checkbox_component :email_opted_in, _("I'd like to receive updates about GitLab via email")

app/views/registrations/welcome/show.html.haml

@@ -17,7 +17,7 @@
         %p.gl-text-center= html_escape(_('%{gitlab_experience_text}. We won\'t share this information with anyone.')) % { gitlab_experience_text: gitlab_experience_text }
       - else
         %p.gl-text-center= html_escape(_('%{gitlab_experience_text}. Don\'t worry, this information isn\'t shared outside of your self-managed GitLab instance.')) % { gitlab_experience_text: gitlab_experience_text }
-      = form_for(current_user, url: users_sign_up_welcome_path, html: { class: 'card gl-w-full! gl-p-5', 'aria-live' => 'assertive' }) do |f|
+      = gitlab_ui_form_for(current_user, url: users_sign_up_welcome_path, html: { class: 'card gl-w-full! gl-p-5', 'aria-live' => 'assertive' }) do |f|
         .devise-errors
           = render 'devise/shared/error_messages', resource: current_user
         .row