Set cache strategy for PWA to show up to date content

[ttraenkler]

Review and set cache strategy used for PWA since by default probably cache first is set and the user will only see new content after opening the webpage twice (cache first strategy). Possibly the strategy could be set for the data/ folder to always pull updated content (network first) but cache first for longer lived assets like images .

• https://github.com/hanford/next-offline#cache-strategies
• https://developers.google.com/web/ilt/pwa/lab-caching-files-with-service-worker
• https://developers.google.com/web/tools/workbox/modules/workbox-strategies

[ttraenkler]

I have verified that the default caching strategy is network first, which should ensure up to date content:
https://github.com/hanford/next-offline/blob/master/packages/next-offline/index.js