Avoid crash when loading STL media without Blender in the server

[rodney757]

So I am moving this here from a merge request, as we are trying to close those out.

I only briefly looked at this, but from what I see is that it is missing a migration, and the WebGL view is not automatically loaded upon visiting the media page.

Here is a link to the original merge request: ​https://gitorious.org/mediagoblin/mediagoblin/merge_requests/57

And to the repo:
​https://gitorious.org/~juca/mediagoblin/mediagoblin-fsanches/commits/conditional_blender_thumbnails

[Rodrigo Rodrigues da Silva]

Can still reproduce the issue as of 76bb92dcb489d490d06993c4023f312aa969ccc2

Loading an STL on a server without Blender installed leads to a crash.

I haven't tested whether this patch still applies, but I don't like the idea of creating a new column blender_thumbs to redundantly represent information that can be retrieved from somewhere else. Especially if we want to plug other renderers in the future, which I think is a good idea.

I'll take a look at it and come up with something soonish.