Decide how to implement PWA features

[ZooeyMiller]

We need to decide how and what PWA features we want to implement.
I'm inclined towards using sw-precache as both @oliverjam and I have experience with it and find the config pretty straightforward.

If nothing else with sw-precache we can at least serve some content when the user is offline, even if the user cannot meaningfully interact with the app whilst offline.
For meaningful interaction we would need to work out how to use localstorage with Elm so we can store our Model offline.

Relates #8

[ZooeyMiller]

We've talked (irl, gasp) about what to do in regards to PWA features, we've decided local storage is a priority.
Considering most people will be using the app on a single device then local storage is pretty important. If we sort local storage then get the communication with the api working to store the model remotely.
We're going to use sw-precache and ports in Elm for local storage.