Sandbox assets w/scripts, resize nicely (#187).

- Add endpoint for rendering asset content in minimal html doc.
- storybase_asset.HtmlAsset.render_html wraps any asset with a script
in an iframe with src pointing to bare asset endpoint.
- Add tests for storybase_asset.HtmlAsset.has_script
- Handle iframe-wrapped asset loading and resizing in both builder
and viewer.
- Unify fix for issue #469 via more general methods in
storybase.views.
- Don't resize container figure element with hard pixel value.
- Bind resize behavior to window resize event. I'm not really sure how
this was working before. The resize event on 'figure img' selectors
was not firing.
- Add removal of window event binding via ViewerApp.remove method. I
don't think this will be called as currently configured, but added for
completeness.
Fix for merge conflict with origin (#187)