Fixed null ref and added % to mark in detailview

See merge request !36

Resolve "Refactoring Marks Module"

Closes #92

See merge request !35

94 course module unit tests

See merge request !34

Merge branch '92-refactor-marks-overview' of https://git.420joos.dev/ost/seproj/studiapp into 92-refactor-marks-overview

Merge branch '92-refactor-marks-overview' of https://git.420joos.dev/ost/seproj/studiapp into 92-refactor-marks-overview

Merge branch '92-refactor-marks-overview' of https://git.420joos.dev/ost/seproj/studiapp into 92-refactor-marks-overview

101 refactoring application

See merge request !33

91 refactoring course module

See merge request !32