Resolve "[Lesson substitution] View with all lesson substitutions in the future"

Closes #11 (closed)

biscuit/apps/chronos/templates/chronos/substitutions.html

+{# -*- engine:django -*- #}
+
+{% extends "core/turnable.html" %}
+{% load bootstrap4 i18n font_awesome %}
+{% load render_table from django_tables2 %}
+
+{% block bootstrap4_title %}{% blocktrans %}Substitutions{% endblocktrans %}{% endblock %}
+
+{% block current_content %}
+  {% render_table substitutions_table %}
+{% endblock %}