Merge metrics branch to dev

[jinnatar]

Metrics seem to be stable enough to merge them to dev. In addition to the metrics themselves this brings in the following noteworthy changes:

• Rudimentary support for loading and using PHP Composer dependencies, although our core code does not use it. The docker images do install the composer dependencies as well.
• Adds the APCu extension to the Docker images. This is required by metrics, but ma also provide interesting opportunities for other improvements as well such as building a unified scheduler.
• Any PDO SQL errors are now thrown as Exceptions instead of silently ignored.
• All exceptions are logged in the error_log, a bigger stack trace is logged if DEBUG is enabled.
• All PHP errors are rethrown as Exceptions so they get the same benefits. This does not include fatal errors though since they're not possible to attach a custom handler to.
• Dropped some old obsolete support for a symlinked core. That hasn't been relevant since we ingested the core.
• Some helpful debug information adds here and there.
• Documentation for Prometheus Metrics and minor improvements to Docker documentation.

[jinnatar]

@Ninjasoturi Any issues left or are we good to merge?

[Ninjasoturi]

I did quick testing to see if existing features work, didn't test anything related to the metrics feature. All basic functions seem to be ok. We'll probably find some well hidden php notices that start to cause issues over time, but good to merge for now.