Use bootstrap buttons instead of input tag.

biscuit/core/templates/contact_form/contact_form.html

@@ -10,6 +10,8 @@
  <form method="post">
   {% csrf_token %}
   {% bootstrap_form form %}
-  <input type="submit" value="Send" />
+  <button type="submit" class="btn btn-secondary">
+    {% blocktrans %}Send{% endblocktrans %}
+  </button>
  </form>
 {% endblock %}