Playground plugin: export data and catch load errors

[bgrgicak]

Fixes #213 #208

What?

This PR adds support for exporting Playground data and implements an error modal when Playground fails to load.

Why?

We would like to allow users to export snapshots of their sites that can later be imported into Playground (a feature in Playground isn't implemented).

Some users reported errors like CORS issues and we need to ensure users have a next step to debug the issue.

How?

Playground snapshot is now an option in Tools > Export.

By catching errors while Playground is loading and displaying an alert.

Testing Instructions

1. Check out the branch.
2. Start a site with the plugin running for example by using wp-env

cd packages/playground
wp-env start

3. Go to wp-admin > Tools > Export
4. Select Playground snapshot and export it
5. Ensure that the downloaded zip contains data
6. Add one of the headers to the top of playground.php

header("Content-Security-Policy: default-src 'self'");

header("Content-Security-Policy: default-src 'self' style-src 'self' 'unsafe-inline' 'unsafe-eval'; script-src 'self' 'unsafe-inline' 'unsafe-eval';");

7. Try starting a sandbox
8. Confirm that you see an error alert

[adamziel]

This could be moved to the login step in core

[adamziel]

It just struck me that it's writing over the WordPress instance that's already in /wordpress. Oh well 🤷 Are we compressing the entire WordPress installation with wp-admin and wp-includes? I wonder if this step would be more appropriate:

    {
      "step": "importWordPressFiles",
      "wordPressFilesZip": {
        "resource": "url",
        "url": playground.zipUrl
      },
      "progress": {
        "weight": 20,
        "caption": "Installing WordPress site"
      }
    }

[adamziel]

Or, even better:

			features: {
				networking: true,
			},
			preferredVersions: {
				wp: playground.zipUrl,
				php: playground.phpVersion
			},

[adamziel]

I left some notes on how to lean more on Blueprints core, but nothing blocking. LGTM 👍

[bgrgicak]

Thanks! Both make sense and we will need to do it as part of enabling imports in Playground.