keep an index of views so that we don't try to re-inject them

fixes #972 

@mattpap I think this will break dev mode, as-is? Can you comment? I suppose we can put the index in its own module with nothing else, and require it in both `main.coffee` and `embed.coffee`.  Or maybe we could use `Collections` for this directly? This PR is just a starting discussion point. 

ping @manderso-brcm