Archive: add assessment documents to course archive

In the assessment tool an assessment document can be added in the assessment form of all assessable  course elements. This document should also be added in the course archive. it needs to be implemented in different ways for all different kinds of archives:

• Test: assessment document is added to every user
• course element: create a zip-file with all assessment documents of the course
• lot files: change of assessment document is already added, but no differentiation between assessment document and comment. --> instead of testcomment, ist should be written assessmentdocument 
• task and group task: add an additional folder 5_assessment and add the assessment document
• checklist: archive of check list results should be possible without adding a file anyway. additionally the assessment document per user is added in the archive

Additionally to the archive, the change log is adapted. Until now for adding an assessment document the log entry "COMMENT set to:" is added. This is changed to "Assessment document added: documentname.pdf"

Intern: CL-846