We were only selectively fixing the mime type for execute_result outputs, now we
treat those the same as we do display outputs. Also fixed image/svg+xml MIME
type wherever I could find it.
Notebooks created directly from ein store image data differently than
notebooks created from the web interface. We now try to fix that when
saving notebooks.
Somewhat hackish solution by defadvicing json.el code to
overcome possible bug in said code. Apparently
`json-encode` does not handle nested empty hash tables properly.
