Backend: Add 401 redirect logic

What does this MR do and why?

Related issue: #2826 (closed)

When the user is not logged in and accessing these two pages ⬇️, redirect to the 401 page:

• Private projects & private groups...
• Page that doesn't exist (like /nothing)

Screenshots or screen recordings

When the user is not logged in and accessing private source or non-exists page:

Before	After

How to set up and validate locally

1. Set SaaS

   export GITLAB_SIMULATE_SAAS=1 
   gdk start

2. Visit a non-exist page like: http://127.0.0.1:3000/nothing

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.