Build as a Progressive Web App

Have the ability to stay offline and send and receive messages when connected to the internet. Keep a cache of all assets.

Slack can be a large pain when in bad reception areas. It would be awesome if our chat did not.

The issue names no files, tests, or entry points. Start by surveying the chat application's existing client and connection handling, then define how offline asset caching and queued message send/receive should work; done means the chat remains usable offline and synchronizes messages when connectivity returns.

Written by the indexing model from the issue text.