Resolve "Improve mobile design of substitution table"

Closes #168 (closed)

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

@@ -80,6 +80,10 @@ table.substitutions td, table.substitutions th {
     padding: 10px 5px;
 }
 
+.overflow-x {
+    overflow-x: scroll;
+}
+
 .lesson-with-sub {
     border: 3px solid red;
     border-radius: 3px;