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)
