Move an entire folder to another assetstore

[manthey]

This would move every file in items in the folder tree to the specified assetstore (the same as file/{id}/move does, but as a single call). We might also want to move associated files (hidden thumbnails, typically) there as well.

[manthey]

This would be a grider endpoint like PUT folder/{id}/move taking an assetstoreId parameter as a first pass, and adding a recurse boolean flag as a second step.

[manthey]

We've had a user report that moving files between assetstores is peculiarly slow. As part of looking into this, we might see if that is the case (e.g., between two filesystem assetstores, we'd expect moving would be only some modest factor over disk access speed).