De-duplicate neuroglancer visualisation

On a dataset page, there are two ways to interact with neuroglancer:

1. The embedded viewer
2. Clicking on "View in neuroglancer"

It would be good to remove one of these so there's not two ways of doing the same thing. If we leave the "View in neuroglancer", I think we should put a box on top of it that users have to click on first with a message (something like "Click for 3D view - requires fast internet"), to avoid Megabytes of data being automatically downloaded every time a user is on a dataset page.