Use btn-dark everywhere. Closes BiscuIT-ng#94.

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

@@ -22,7 +22,7 @@
   <form method="post">
     {% csrf_token %}
     {% bootstrap_form edit_substitution_form %}
-    <button type="submit" class="btn btn-secondary">
+    <button type="submit" class="btn btn-dark">
       {% blocktrans %}Save{% endblocktrans %}
     </button>
   </form>

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

@@ -15,7 +15,7 @@
     <ul id="timetable_select_form">
       {{ select_form.as_ul }}
     </ul>
-    <button type="submit" class="btn btn-secondary">
+    <button type="submit" class="btn btn-dark">
       {% blocktrans %}Select{% endblocktrans %}
     </button>
   </form>