Improvements for GitLab.com subscriptions

• Remove Sign up buttons from /gitlab-com
• Remove GitLab.com options from /products and link to /gitlab-com

Review:

• https://gitlab-com-changes.about.gitlab.com/products/
• https://gitlab-com-changes.about.gitlab.com/gitlab-com/

Closes https://gitlab.com/gitlab-com/www-gitlab-com/issues/1324