Make page titles and headers consistent. Advances BiscuIT-ng#65.

biscuit/apps/chronos/templates/chronos/edit_substitution.html

 {% extends "core/base.html" %}
 {% load bootstrap4 i18n %}
 
+<title>{% block bootstrap4_title %}{% blocktrans %}BiscuIT SIS - Add book{% endblocktrans %}{% endblock %}</title>
+
 {% block page_title %}{% blocktrans %}Edit substitution{% endblocktrans %}{% endblock %}
 
 {% block content %}