Export by identity

Description

Extends the exporting features to export all submissions for an ID, globally or within an assignment.

Additions

• A new endpoint adding an identity filter to the already existing per-assignment submission endpoint
• A new endpoint accepting an identifier instead of an assignment ID to export everything linked to that identity
• A findByIdentifierEquals function for IdentityRepository

Changes

(in order of appearance)

• Renamed AssignmentExportController to SubmissionExportController since that reflects the meaning a bit better
• Assignment names are now filtered for all invalid filename characters instead of just quotes
• The assignment ID is also included in the export
• To generalize the code as much as possible, assignment and aid parameters are now nullable in many places
• The API docs table of contents is now scrollable

Test and Review

Closes #171 (closed)

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