Stricter PHP quality tests in GitLab CI

Problem/Motivation

Currently GitLab CI carries out code quality tests, but they are not required to pass.

Also, phpstan runs at the default level "1". We can do better!

Steps to reproduce

Make a change that causes phpcs or phpstan tests to fail. GitLab CI will report a warning, but will not stop build passing.

Proposed resolution

Set allow_failure to false for phpcs and phpstan.

Also add a phpstan.neon file to specify the phpstan level.

Remaining tasks

Add allow_failure variables to .gitlab-ci.yml
Create phpstan.neon file specifying level 5

User interface changes

N/A

API changes

N/A

Data model changes

N/A