Disable or clarify caching for printables

[alexanderkoller]

Currently, browsers tend to cache the PDF files that need to be printed. This can be confusing to users, who expect to change the PDF right away after they have made edits.

Preferrably, disable caching of such PDFs (see e.g. https://stackoverflow.com/questions/34066804/disabling-caching-in-flask).

Until this is done, put a note on the admin page that explains about shift-reload.