Add service worker for offline caching

[Gudahtt]

A service worker has been added that caches the phishing warning page. This ensures that it remains functional if GitHub pages goes offline.

The service worker precaches all CSS, HTML, SVG, and JavaScript files. These are the files required for the site to function properly. The fonts and the .ico favicon used by older browsers are omitted to reduce the cache size.

I did attempt to add a runtime cache for the fonts, but I could never get it to work correctly so I abandoned the effort. It's not critical, the fallback web font looks fine.

[danjm]

LGTM

[Gudahtt]

Thanks Dan! Sorry I marked this as ready for review prematurely, I meant to add more docs and QA instructions. I'll do that now

[Gudahtt]

To QA offline support, I used these steps:

• yarn setup
• yarn build
• npx static-server ./dist
• Navigate to the page and open dev tools
• Disable network requests (by selecting "Offline" in the throttling menu on the Chrome dev tools network tab, or by adding a * entry to the request blocking list on Firefox) and hard-reload the page.

Previously, the page load would fail after disabling network requests. After this PR, the page should render correctly after a second or so, after the browser lets the network request timeout. It does initially display a "failed to load" page until that second elapses.

To test that the service worker updates correctly and doesn't keep stale assets around:

• yarn setup
• yarn build
• npx static-server ./dist
• Navigate to the page
• Double-check that the assets are cached by looking in dev tools (It's under the Application tab on Chrome, under "Cache storage").
• Stop the server
• Modify a static asset (e.g. change the text in the HTML file)
• yarn build
• npx static-server ./dist
• Hard-reload the page

It should load the updated asset correctly.