Add a checkstyle job to the CI pipeline

Description

Reintroduces Checkstyle to the continuous integration pipeline. The herokuish buildpack only runs tests, so we had a lot of unexpected failures locally.

Changes

None

Additions

• A checkstyle job in the review stage. It is not allowed to fail. In a separate stage so you have to wait just to annoy @eoudsen

Test and Review

To be filled in by the reviewers

• There are no unnecessary files present in the MR

• The continuous integration has no problems with the MR

• The MR is filled in as requested (including labels, milestones, and reviewers)

• The documentation is up-to-date