remove application/pdf->pdf transform in javascript

We have the old transforms because they were added before we realized it was a bad idea. We should not add new ones.
handle 'application/pdf' in nbconvert

since we shouldn't add a 'pdf' shortcut
remove inappropriate 'pdf' key

in favor of 'application/pdf'
bibtext typo
fix file extension when extracting PDF figures
add pdf to extract output tests
add pdf output to test notebook
fix check for non-attr output keys
Merge pull request #5132 from minrk/oops-pdf-transform

remove application/pdf->pdf transform in javascript