PWA offline mode

[yujqiao]

It will be great if

1. PWA can be used without network connection
2. User can choose to cache a certain book. Potentially a number of pages.

The use case is that when I'm away from home, my reader usually doesn't have network.

[majora2007]

I'm open to a PR around PWA, but I'm personally planning to build a native app for Kavita so that I can have the vision I've been envisioning since day 1.