Merge branch 'core-80-use-icons-instead-of-plain-text' into 'master'

Use icons instead of plain text. Advances BiscuIT-ng#80.

See merge request BiscuIT/BiscuIT-App-Chronos!27

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

 {# -*- engine:django -*- #}
 
 {% extends "core/base.html" %}
-{% load bootstrap4 font_awesome i18n %}
+{% load bootstrap4 i18n %}
 
 {% block bootstrap4_extra_head %}
   {{ block.super }}
@@ -24,7 +24,7 @@
       {% if substitution %}
         {# FIXME Respect year as well #}
         <a href="{% url 'delete_substitution' substitution.lesson_period.id substitution.week %}" class="btn btn-danger">
-          {% fa 'trash-o' %}
+          <span class="mdi mdi-delete"></span>
         </a>
       {% endif %}
     </div>

biscuit/apps/chronos/templates/chronos/lessons_day.html

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

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

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

biscuit/apps/chronos/templates/chronos/tt_week.html

 {# -*- engine:django -*- #}
 
 {% extends "core/turnable.html" %}
-{% load bootstrap4 data_helpers font_awesome i18n staticfiles %}
+{% load bootstrap4 data_helpers i18n staticfiles %}
 
 {% block bootstrap4_extra_head %}
   {{ block.super }}