Add session state store and event bus types

[flyingrobots]

Summary

Set up some kind of local storage-based state system to store demo state for the Playground. Look into setting up an event bus for this page for a dynamic, reactive UX.

User Story

As a	Playground user
I want	My browser to remember stuff and be able to handle events from Postgres
So that	I can tell if we're connected to Postgres and my browser will remember my settings if I come back later

Acceptance Critiera

• Event bus exists
• Ability to send events to the bus and see the bus handle them (debug print OK)
• Ability to store persisted data between page loads locally in-browser (debug print OK)

In-Scope

• Sending events
• Subscribing to events
• Write local data
• Read local data

Out-of-Scope

• Hooking this up to anything in particular
• Routing Postgres events