Fixes for bootstrap4/admin/lib.html

Boostrap 4 renamed .control-label to .col-form-label.
https://getbootstrap.com/docs/4.0/migration/#forms-1

flask_admin/templates/bootstrap4/admin/lib.html

@@ -120,9 +120,9 @@
   {% set prepend = kwargs.pop('prepend', None) %}
   {% set append = kwargs.pop('append', None) %}
   <div class="form-group {{ kwargs.get('column_class', '') }}">
-    <label for="{{ field.id }}" class="control-label {% if field.widget.input_type == 'checkbox' %}d-block mb-0{% endif %}">{{ field.label.text }}
+    <label for="{{ field.id }}" class="col-form-label {% if field.widget.input_type == 'checkbox' %}d-block mb-0{% endif %}">{{ field.label.text }}
         {% if h.is_required_form_field(field) %}
-          <strong class="text-danger"">&#42;</strong>
+          <strong class="text-danger">&#42;</strong>
         {%- else -%}
           &nbsp;
         {%- endif %}