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

assigned to @leverse

added 1 deleted label

marked the checklist item There are no unnecessary files present in the MR as completed

marked the checklist item The continuous integration has no problems with the MR as completed

marked the checklist item The MR is filled in as requested (including labels, milestones, and reviewers) as completed

marked the checklist item The documentation is up-to-date as completed

approved this merge request

approved this merge request

changed the description

merged

mentioned in commit 98a1b762