Write and update a */wp-content/maintenance.php* file during the core/plugin/theme/language update and install steps; and run those steps in a background process, while the FastCGI server starts-up ASAP. - [ ] ~~Ensure writing said file works as intended; returns 503 and content~~ - [ ] ~~Background install steps à la wp-cron~~ - [ ] ~~Set caching headers to disable caching?~~ - [x] Build a page including a spinner and self-updating mechanism (meta-redirect?)