(optim): add PWA support with OfflinePlugin

https://github.com/NekR/offline-plugin supports Service Workers and falls back to AppCache

I haven't read too far in-depth into exactly how SWs work, so not sure of all the possible concerns. Off the top of my head of some issues I've previously read about are cache-busting the SW, excluding/including certain requests, and figuring out how caching app state works in tandem with the SW are all important considerations. Also not sure how useful SWs are to apps that don't have very much offline functionality; I think the caching aspect might still work better for online apps than just plain browser caching, but not really sure of all these details.

An alternative is Google's [Workbox](https://github.com/GoogleChrome/workbox), which is the successor to [sw-precache-plugin](https://github.com/goldhand/sw-precache-webpack-plugin). Based on the docs and [this chat thread](https://spectrum.chat/thread/1955e8fd-5c44-4e0a-acd9-8046e53739e2), offline-plugin is much easier to set-up and get started with.