Support configuring GitLab with a relative URL root

Summary

GitLab chart is currently limited to deploying GitLab at the root path of a domain or subdomain. Meanwhile Omnibus and source installation methods also offer the ability to configure a relative_url_root, allowing GitLab to be deployed at a custom path (such as source.example.com/gitlab).

To maintain feature parity across all installation methods, GitLab chart should also support this functionality. Optimally, this can be configured by setting a single (global) value.

Links

• Relative URL for source installs: https://docs.gitlab.com/install/relative_url/
• Relative URL for Omnibus installs: https://docs.gitlab.com/omnibus/settings/configuration/#configure-a-relative-url-for-gitlab
• Warning about using relative installs: https://gitlab.com/gitlab-org/gitlab/-/blob/70958ddf3be2de08f204dcf8c02dbeb87a44c69f/config/gitlab.yml.example#L51
• MR that added support in Omnibus: Relative url support for omnibus installations (gitlab-org/omnibus-gitlab!590 - merged)

Acceptance Criteria

• GitLab can be deployed with a relative URL root
  • Rails configured for relative URL root
  • Workhorse configured for relative URL root
  • GitLab shell configured for relative URL root
  • Gitaly configured for relative URL root
  • Set RAILS_RELATIVE_URL_ROOT for sidekiq mailers (gitlab-org/omnibus-gitlab!1381 (merged))
• Ingress changes implemented (if necessary)
• Probe changes implemented (if necessary, probably not needed per !4462 (comment 2685814895))