Epic: Add PWA install banners, analytics and offline fallback page

[annarhughes]

Overview

This epic aims to improve our promotion of Bloom being available to install/download as a Progressive Web App (PWA). The epic breaks down UI and analytics tasks into sub issues, one for each new feature. Each sub issue contributes towards improved UX and likelihood of users returning to Bloom and having super easy access to our content 😌

Read more about PWA and it's capabilities here and here. The core offering our PWA has at the moment is (minimal) offline support and the app being added the Bloom app to the users homescreen. We may add more PWA features in the future but these core features are already valuable, so we want to promote them to users.

Our current PWA implementation:

• Basic PWA functionality is set up using next-pwa
• The PWA can be downloaded/installed from the browser. This feature is not promoted - that's what this epic aims to fix.
• Push notifications and offline fallback page is not set up

Action Items

Each sub issue should be completed on a separate PR. #1211 and #1212 can be done asynchronous and are not blocked by other sub issues. These issues need to be completed sequentially with this ordering #1207 #1209 #1208 . It may be helpful to await the approval of the previous issue before starting the next.

• Add PWA download banner to mobile navigation #1207
• Add PWA download banner for desktop course page #1209
• Add PWA analytics events #1208
• Add PWA detection analytics event #1211
• Add PWA offline fallback page #1212 (blocked by design)