Adding a service worker and manifest file to Gitlab to qualify it as a basic progressive web app

What does this MR do?

This ticks all the necessary boxes to register gitlab.com (or anything running GitLab) as a PWA.

It's definitely the most boring solution as all it does is:

1. Create an "Offline" page at /offline.html

2. Register a Simple service worker that caches the "Offline" page

3. Serves up the "Offline" page when a user is offline. It also has the manifest stuff necessary for setting up a PWA. It's not wholly useful as it is, but it's a good jumping off point for getting some offline support and caching.

There's still some work to be done to get it into a nicer workflow and we'll probably need to look at the copy/i18n on the offline page too.

Here's how to get it runnning locally: https://www.youtube.com/embed/V93cVpOGoCQ

What are the relevant issue numbers?

closes #49108 (moved)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer