Add better options to get previous or next lesson period

• Add properties to LessonPeriod model
• Also annotate year while annotating a week
• Fix use of negative indexes in next_lesson

aleksis/apps/chronos/models.py

@@ -335,7 +335,7 @@ class LessonPeriod(ExtensibleModel):
         """Get previous lesson period of this lesson.
 
         .. warning::
-            To use this property a week had to be annotated to the provided lesson period.
+            To use this property,  the provided lesson period must be annotated with a week.
         """
         return LessonPeriod.objects.filter(lesson=self.lesson).next_lesson(self, -1)