WordPress feature parity

[adamziel]

Let's make sure that WordPress Playground can do everything a regular WordPress can

Features

• Enable networking by default #1225
• [Database] Exported and hosted Playground site can't connect to Jetpack #708
• Document Custom WordPress builds #740
• Multisite mode #499

Stability improvements

• Pass WordPress unit tests #111
• Error enabling "Custom fields" panel in post editor preferences #645
• Missing CSS and images because the Gutenberg iframe isn't controlled by the service worker #646
• Run regular integration test against developer.wordpress.org #599
• Remove use of WP_DEVELOPMENT_MODE constant #627
• Document more visibly how to set the iframe to full width #722
• Include /wp-includes/fonts/dashicons.svg in the bundle #729