Bugfixes in worker management

Description

Resolves #136 (closed) and #107 (closed).

Changes

Solves #136 (closed) by running report generator in a separate thread and #107 (closed) by checking if submissions have a CPM token.

Test and Review

Fixed a faulty test checking for a wrong date.

To be filled in by the reviewers

• All of the methods are commented to expectation

• The methods are tested to satisfaction

• 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

• All nullable parameters are marked as such