feat: locking days, if they are submitted and accepted

fixed #76

Backend/models/iworkday.go

@@ -23,6 +23,7 @@ type IWorkDay interface {
 	GetTimes(User, WorktimeBase, bool) (work, pause, overtime time.Duration)
 	GetOvertime(User, WorktimeBase, bool) time.Duration
 	IsEmpty() bool
+	IsSubmittedAndAccepted() bool
 }
 
 type DayType int