Adds a slot for account button in play-pen-header, that can be specified via play-pen.

## 💸 TL;DR

Implements Project-level saving and loading in Play.

## Details

This includes:
* Adding the Save/Load buttons in the Project menu
* Dialogs for both Save and Load flows
* Logic that proxies the actual storage to something behind the
`ProjectStorageClient` interface, which can be swapped out later
* Storing the current project in `sessionStorage`, so that it persists
across reload but not across tabs
* Making sure "New" projects clear the current stored project
* Making sure we can load/save projects several times
* Using the Pen title as the project name (both when saving and loading)