Default all responses to 500 before any code is run so PHP will throw 500's on FATAL errors

Problem/Motivation

If PHP throws a FATAL error, a 200 response will be returned.

This could mean that the 200 'OK' response gets cached by a browser, a reverse proxy, or a CDN.

Proposed resolution

I was sure PHP could return a 500 is maybe display_errors were turned off, but from testing this wasn't happening for me.

If we set the status code to 500 in index.php, before any other code is run, we can ensure that PHP will return a 500 response code for FATAL errors. All other cases will be handled by a Response object, so will have the correct/intended status code

Remaining tasks

User interface changes

API changes

Data model changes