Add PWA offline fallback page

[annarhughes]

Overview

Bloom works as a PWA app so users can install/download as a browser app. See #1197 for more details on PWA.

This task involves building an offline fallback page to be shown when the app is offline and no cached content is available for that page.

Action Items

• Build the offline page according to the design
• Ensure all translations for the text on the page are added to /messages/
• Ensure the offline fallback page is setup correctly

Resources/Instructions

Setting up offline fallback page with next-pwa - link
PWA offline mozilla guide - https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps/Guides/Offline_and_background_operation

Design