Fixes the direct upload endpoint to work wih trampoline again.

Description

Closes #188 (closed)

Changes

Changed direct upload endpoint to return an id of the newly created submission

Test and Review

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