Add a sandbox mode for experimentation

Add an optional sandbox mode in which users can browse and install themes and plugins to experiment with.

• Add some way for sandbox mode to be enabled that is not easy to do in ignorance.
• Make WP_CONTENT_DIR writable for the PHP-FPM user and disable the filter which intercepts user write-permissions checks.
• Trigger the entrypoint's static file gather function after (un)install.
• Add BDD tests for sandbox/regular mode installs (or attempts to install)
• Document how to enable the mode and when to (not) enable it.