Add missing PDF dependencies to autograder zip files

[chrispyles]

The changes made for #458 involved making some dependencies required for Otter Export not part of the library's install_requires, meaning these libraries now need to be added to the requirements.txt file created by Otter Generate (e.g. pdfkit).

[chrispyles]

Rather than updated the requirments.txt file (since we don't use html exports in the grading image), I'm going to change it so that the otter.export.exporters.via_html module is only imported once it's been determined that an html export is needed.