3911 do not render empty lists on article page

[StephDriver]

Updated django templates to check that there are galleys before creating download and checksum lists, across all three themes. Material already checked for galleys for download lists, but not for checksums.

For empty autogenerated tables of contents in OLH theme, updated the JS to remove the toc element if no headings were found, i.e. if list not populated.

Marked as draft as it is the end of the day, and I have yet to test this, so that must be done before sending for review.