is_valid attribute is not needed anymore

[capooti]

We can get rid of this attribute.

We can now associate an issue to any Layer/Service which is problematic. We should have a script which create issues for service/layers which are now marked as not valid.

When harvesting the service/layer, if there are conditions that previously marked the service/layer as invalid, they should now associate an issue to the same service/layer.

[capooti]

The script which will create issues should become a celery task

[capooti]

@thaorell can you work on this as well?

Basically, the update_validity methods for Service and Layer in aggregator/models.py are obsolete, as now we use the Issue model to set a Service or Layer which is problematic.

The update_validity code should be now be part of a new Celery task, let's name it set_invalid_layers, which, running on a periodic basis, will set an issue for layers when:

• layer is not in a supported SRS
• layer is part of WMTS
• layer is part of a service which contains 'noaa' in url
• layer falls within an extent which is close to 0, 0