Let users install OT as a Progressive Web App (PWA)

[hathix]

PWAs are all the rage, and they let you create "apps" without having to build a unique native app for each platform (which we don't have the energy to do).

This site has a checklist for how to make our app installable as a PWA.

I think the biggest challenge is being able to display some information offline. We're not currently set up for that at all. Everything else looks pretty easy, or we've done it already.