Group and uniform business code handling celery task results

[c8y3]

In app.blueprints.rest.dim_task_routes celery tasks results are retrieved in the database.
In app.blueprints.pages.dim_tasks.dim_tasks, the task result is retrieved from its identifier thanks to the Celery AsyncResult object.
It would improve code regularity to access dim tasks results always through the same mechanism (most probably always going through the database, as Celery does not provide any public API to list all task results) and group the code down into the business layer.