Resource update: last_modified research

[danmihaila]

In CKAN 2.5.1 the last_modified field is getting updated when doing an upload via UI. We also need to check the API upload.
Data team needs this field to be updated when a resource is uploaded/created/modified/etc.
The main task is to check if we can bring the code from 2.5.1 into our version or to make our own action.
Also, after research we could use "revision_timestamp" from resource_revision to update the field with last action for a resource.

[alexandru-m-g]

@danmihaila I've tested a bit on the CKAN beta site and it doesn't seem like the last_modified property is being updated. Check this link: http://beta.ckan.org/api/action/package_show?id=this-is-just-a-test-dataset

I see that there is a case in which uploaded files get that field populated here but it doesn't seem to be the "default" case. Also, it doesn't deal with API resources in any way as far as i see.

I think, we could change the validation step to update that field automatically.