Fixed error messages not showing in formulars

This is a known unresolved issue in Bootstrap 4
See: twbs/bootstrap#23454

Author: fallen

pytition/petition/templates/petition/bs4_form.html

@@ -23,7 +23,7 @@
           {% render_field field|bootstrap class="is-invalid" %}
         </div>
         {% for error in field.errors %}
-          <div class="invalid-feedback">
+          <div class="invalid-feedback d-block">
             {{ error }}
           </div>
         {% endfor %}