Feature - Assignment exports

Description

Introduces an API endpoint for full assignment exports (that is, exports of all the submissions for this assignment). It supports both JSON and CSV output format, the latter of which supporting even more sub-formats provided by Apache Commons CSV.

Changes

None.

Additions

• An export controller
• An exception and error code for unknown formats
• In test-land, a helper class creating mock repositories with sane defaults to make testing somewhat easier

Test and Review

Closes #172 (closed)
Depends on !79 (merged)

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