Resolve "Check permission rules if they handle lessons with substitutions correctly"

Closes #147 (closed)

get_teachers will return the following:

• simply the teachers for events and extra lessons
• the original teachers for lesson periods without substitution, otherwise the new teacher

So, we have to check the teachers of lesson periods additionally to keep the access for the original teacher.

aleksis/apps/alsijil/preferences.py

@@ -34,6 +34,14 @@ class RegisterAbsenceAsPrimaryGroupOwner(BooleanPreference):
     )
 
 
+@site_preferences_registry.register
+class EditLessonDocumentationAsOriginalTeacher(BooleanPreference):
+    section = alsijil
+    name = "edit_lesson_documentation_as_original_teacher"
+    default = True
+    verbose_name = _("Allow original teachers to edit their lessons although they are substituted")
+
+
 @site_preferences_registry.register
 class CarryOverDataToNextPeriods(BooleanPreference):
     section = alsijil