Cherry picks from 5.1

Michael Jansen authored Feb 16, 2018 and Niels Theen committed Mar 22, 2018

(cherry picked from commit 930346c1)

Michael Jansen authored Jan 15, 2018 and Niels Theen committed Mar 22, 2018

(cherry picked from commit 5d010c81)

(cherry picked from commit 8177fd6a)

Michael Jansen authored Jan 15, 2018 and Niels Theen committed Mar 22, 2018

(cherry picked from commit 239f0e43)

(cherry picked from commit 5dd97309)

0022499: Vormals aktiver Fragenfilter wird nach späterem Neustart desselben CTM-Tests nicht angezeigt

fixed mantis #22186 - the followup issue // when multi values are ENABLED and ->checkInput is NOT called there is no post parameter available WHEN the selection is left empty

fixed mantis 0021868: Failed test: Jeden Fragentyp in der Listenansicht hinzufügen // essay questions keyword relation migration was not complete - repaired broken questions with HOTFIX 5.0

fixed mantis 0010711: Nach Import eines Fragenpools fehlt antwortspezifisches Feedback in der Fehlertextfrage (...)

Michael Jansen authored Oct 17, 2017 and Bjoern Heyser committed Nov 23, 2017

Fixed #21627 / \ilSoapLearningProgressAdministration::deleteProgress: Call to an undefined method ilObjTest::removeTestResultsForUser

fixed (hopefully all) image scr conmversions as well as linebreak issues in toJSON per qtype (mantis 20325)