Add switch posterSession for virtual poster session json file generation

Add switch posterSession to add feature where json and csv files with all poster URLs are created, for use in virtual poster sessions (where you need to upload the latest versions of all the posters really fast - we used it with the Gather.Town API for TIPP21 and it worked rather well. Sample usage file available on request). In order not to change the way it works on the day the conference starts, it sets "inThePast=False" when it runs, so it will always find the empty poster contributions as well as the ones with uploaded PDF files, and will not mess with any placeholders you have set up.