Allow docsets to enable online resource access

[trollixx]

Through multiple back and forth changes, I believe the current logic blocking download of online resources from local pages is a sensible default. At the same time there are use cases when online access is desirable. One was mentioned in the very old #474.

One option would be adding support for DashDocSetBlocksOnlineResources, although I believe Dash uses it in an opposite way.

Another way would be allowing end users to enable online access for a particular docset. That should provide a workaround for issues like #1302, until docsets are fixed upstream.