Add header box with affected teachers and groups in substitution views

• On/off switchable via constance
• Included in substitutions.html and substitutions_print.html
• Add extra QuerySet functions (affected_lessons, affected_groups, affected_teachers)

Advances #61 (closed)

aleksis/apps/chronos/static/css/chronos/timetable.css

@@ -108,3 +108,7 @@ table.substitutions td, table.substitutions th {
     margin: 2px;
     letter-spacing: 0.3pt;
 }
+
+.black-text-a a {
+    color: black;
+}