Support DICOMWeb assetstores

[manthey]

With girder/large_image#1303, we now have a DICOMweb assetstore. We want imports from these to be tracked. We should also add the Exclude Existing Files option to that import.

[manthey]

There are a number of open issues on expanding some of what the DICOMweb assetstore will do (e.g., there may be filters on what gets imported), so it is possible that any solution will have to update alongside those changes.