Feat: add unauthorized page

What does this MR do and why?

This MR is used to add a 401 page.

Related issue: #2826 (closed)

backend MR: !1660 (merged)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Set SaaS

export GITLAB_SIMULATE_SAAS=1 
gdk start

2. rebase !1660 (merged)
3. Visit a non-exist page like: http://127.0.0.1:3000/nothing

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

/cc @mtan-gitlab-cn