fix 'Static call to instance method' errors

db/migrations/1.231_add_files_search_index.php

lib/classes/StudipObject.class.php

lib/evaluation/classes/EvaluationExportManagerCSV.class.php

lib/evaluation/classes/EvaluationObject.class.php

lib/evaluation/classes/db/EvaluationAnswerDB.class.php

lib/evaluation/classes/db/EvaluationDB.class.php

lib/evaluation/classes/db/EvaluationGroupDB.class.php

lib/evaluation/classes/db/EvaluationObjectDB.class.php

lib/evaluation/classes/db/EvaluationQuestionDB.class.php

lib/evaluation/evaluation_admin_edit.inc.php

lib/evaluation/evaluation_admin_edit.lib.php

lib/evaluation/evaluation_admin_overview.inc.php

lib/evaluation/evaluation_admin_overview.lib.php

lib/evaluation/evaluation_admin_template.inc.php

lib/evaluation/evaluation_admin_template.lib.php

lib/extern/lib/ExternError.class.php

lib/extern/lib/ExternModule.class.php

lib/models/Courseware/Bookmark.php

lib/models/CronjobTask.class.php

lib/raumzeit/IssueDB.class.php