Merge branch 'core-116-migrate-to-material-design-icons' into 'master'

Use material-design-icons instead of font_awesome. Advances BiscuIT-ng#116.

Closes BiscuIT-ng#116

See merge request BiscuIT/BiscuIT-App-Untis!10

biscuit/apps/untis/templates/untis/untis_import.html

 {# -*- engine:django -*- #}
 
 {% extends "core/base.html" %}
-{% load bootstrap4 font_awesome i18n static %}
+{% load bootstrap4 i18n static %}
 
 {% block bootstrap4_title %}{% blocktrans %}Import Untis data{% endblocktrans %} - {{ block.super }}{% endblock%}
 
@@ -15,7 +15,7 @@
       {% endblocktrans %}
     </p>
     <div class="alert alert-danger" role="alert">
-      {% fa 'exclamation-triangle' %}
+      <span class="mdi mdi-alert"></span>
       {% blocktrans %}
         Newly imported data will be valid as of tomorrow.
       {% endblocktrans %}