Editing of default viewpoint

[andybak]

Our megajson will seed ex-poly assets with a (hopefully) correct camera position.

I think I can extract info from GLTFs for some non-poly assets.

Moving forward the upload code should set this more clearly and the publish screen will allow this to be tweaked.

the "edit asset" page should also allow this to be modified. It should be as simple as a button that says something like "use current viewpoint as default". Maybe needs to update thumbnail as described in #53 at the same time?