Plugin auto migrate (#5668)

* Improved handling of race condition when saving setting value

* Improvements for managing pending migrations

- Inform user if there are outstanding migrations
- reload plugin registry (if necessary)

* Increase django-q polling time

According to this thread, should reduce multiple workers taking the same task:

https://github.com/Koed00/django-q/issues/183#issuecomment-239676084

* Revert default behavior

* Better logging

* Remove comment

* Update unit test

* Revert maintenance mode behaviour

* raise ValidationError in settings

InvenTree/templates/base.html

@@ -8,6 +8,7 @@
 {% settings_value 'RETURNORDER_ENABLED' as return_order_enabled %}
 {% settings_value "REPORT_ENABLE" as report_enabled %}
 {% settings_value "SERVER_RESTART_REQUIRED" as server_restart_required %}
+{% settings_value "_PENDING_MIGRATIONS" as pending_migrations %}
 {% settings_value "LABEL_ENABLE" as labels_enabled %}
 {% inventree_show_about user as show_about %}
 
@@ -106,6 +107,16 @@
                     </small>
                 </div>
                 {% endif %}
+                {% if pending_migrations > 0 %}
+                <div id='alert-pending-migrations' class='alert alert-danger' role='alert'>
+                    <span class='fas fa-database'></span>
+                    <strong>{% trans "Pending Database Migrations" %}</strong>
+                    <small>
+                        <br>
+                        {% trans "There are pending database migrations which require attention" %}. {% trans "Contact your system administrator for further information" %}
+                    </small>
+                </div>
+                {% endif %}
             </div>
             {% endblock alerts %}