Setup for offline route as a start for the PWA

What does this MR do and why?

So we want to setup a PWA infrastructure to especially have a service worker in the background for this domain to pre-cache data. This as a starting point adds the new offline page route which then can be downloaded by the service worker to show when the user or GitLab is offline. The full setup will look something like this - !56873 (closed) and changes are split up to have easier iteration and finding the problem with constant failing tests.

Screenshots or screen recordings

How to set up and validate locally

Review App route of offline page - https://gitlab-review-tz-add-off-m5yavn.gitlab-review.app/-/offline

MR acceptance checklist

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