Add an optional sandbox mode in which users can browse and install themes and plugins to experiment with. - [x] Add some way for sandbox mode to be enabled that is not easy to do in ignorance. - [x] Make `WP_CONTENT_DIR` writable for the PHP-FPM user and disable the filter which intercepts user write-permissions checks. - [x] ~~Trigger the entrypoint's static file gather function after (un)install.~~ - [ ] Add BDD tests for sandbox/regular mode installs (or attempts to install) - [x] Document **how** to enable the mode and **when** to (not) enable it.