ITK_VIEWER_SRC is hard-coded to an IPFS address

[bcdarwin]

My institution blocks IPFS addresses, so I can't use itkwidgets for work at all (manifests as a blank/crashed box as shown in #769) - would be nice if there were an alternate way to load the viewer without rebuilding/hacking the wheel.

[thewtex]

@bcdarwin thanks for the feedback.

We are moving towards a the bundling of all assets.