Resolve "Summarise double lessons"

Closes #97 (closed)

aleksis/apps/chronos/templates/chronos/partials/subs/period.html

 <strong>
-  {% if type == "substitution" %}
+  {% if type == "substitution" and item.start_period == item.end_period %}
     {{ el.lesson_period.period.period }}.
+  {% elif type == "substitution"  %}
+    {{ item.start_period }}.–{{ item.end_period }}.
   {% elif type == "extra_lesson" %}
     {{ el.period.period }}.
   {% elif type == "event" %}