Use PHP FPM

If we use PHP FPM instead of mod_php in Apache we get:

* Ability to run the PHP process as a different user
* Ability to run multiple versions of PHP (maybe?)
* Better separation of concerns
* A less familiar setup, but one that we probably need to get comfortable with

A little todo list:

- [x] Install the relevant PHP-FPM package
- [x] Change the Apache config to use mpm worker.
- [x] Change the vhost template to use PHP-FPM
- [x] Configure FPM to run as the same user as the host OS.
- [x] Make sure timeouts are large enough to debug etc.
- [x] Stress test the server
- [ ] Document this