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

biscuit/apps/alsijil/templates/alsijil/lesson.html

@@ -78,7 +78,7 @@
       </div>
     </div>
     <button type="submit" class="btn btn-dark">
-      {% blocktrans %}Save{% endblocktrans %}
+      <span class="mdi mdi-content-save"></span>
     </button>
   </form>
 {% endblock %}