Git commits in Moodle x.x.x by Adrian Perez

commit ac33483290931f8d6cef8f81cee46392310e55f2
Author: Adrian Perez <adrian.perez@ffhs.ch>
Date:   Fri, 20 Jul 2018 20:25:25 +0800
Tag:    v3.6.0-beta

MDL-63002 core_grade: fixed deletion of overriden letters

commit 10dae695ecc5669ebae9f17e8ecf1fe5150e1720
Author: Adrian Perez <adrian.perez@ffhs.ch>
Date:   Fri, 20 Jul 2018 20:25:25 +0800
Tag:    v3.5.2

MDL-63002 core_grade: fixed deletion of overriden letters