Implement Purge action to delete repositories

[pdelboca]

CKAN has an action called dataset_purge that we can be leverage to implement the deletion of repositories. The behaviour should be:

• When deleting a dataset, do nothing with the repo (We theoretically could put it into archive status but then we cannot undo that action through the API)
• When purging the dataset, delete the repo.

Note:

• Even when this action exist, the current trash action of the admin controller does not use it. This action is only used by the purge cli command.
• We may need to write our own trash view to use this action.

[shevron]

@pdelboca what is the behavior expected by users? If currently this is only exposed in CLI, shouldn't we keep it that way?

BTW the action can also be used via HTTP using an admin API key + POST.