Add a blueprint to Gutenberg plugin space

[bph]

The Gutenberg plugin doesn't have a Playground preview button in the WordPress Repo, and it might help users trying out the latest Gutenberg version via playground.

The plugin review team published documentation on how to add a blueprint.json to the plugin SVN space:

There are two things required for a plugin preview button to appear to all users:

A valid blueprint.json file must be provided in a blueprints sub-directory of the plugin’s assets folder. /assets/blueprints/blueprint.json
The plugin preview must be set to “public” from the plugin’s Advanced view by a committer.

The blueprint would look like this: (updated from the PR)

{
	"$schema": "https://playground.wordpress.net/blueprint-schema.json",
	"landingPage": "/wp-admin/post.php?post=1&action=edit",
	"plugins": [ "gutenberg" ],
	"login": true,
	"features": {
		"networking": true
	},
	"preferredVersions": {
		"php": "latest",
		"wp": "latest"
	},
	"steps": [
		{
			"step": "setSiteOptions",
			"options": {
				"blogname": "Testing Gutenberg"
			}
		},
		{
			"step": "updateUserMeta",
			"meta": {
				"admin_color": "modern"
			},
			"userId": 1
		}
	]
}

The plugin page lists three contributors, I assume are the committers, who need to do step two. @mtias @jasmussen or @karmatosed