Rename project slug

Problem

This is minor but using the domain name as the project slug makes searching browser history less accurate and effective

Proposal

Let's rename the project slug to design-gitlab-com.

Benefits

1. Separate browser history items from the repository (design-gitlab-com) and the deployed app (design.gitlab.com).
2. Remove the need to mention the repository using capital 'G' or 'L'.
3. Follow the pattern used for other repositories deployed on a specific URL like version-gitlab-com, license-gitlab-com, www-gitlab-com, and www-remoteonly-org.