Generate and merge json file for participants file

[tsalo]

BIDS now likes to have json files for its tsv files, so we should generate one for the participants file. We can copy it over if it doesn't exist in the centralized dataset, and skip if it does.

[akimbler]

@tsalo Unless I'm crazy, the newest version of heudiconv does this automatically, we just need to update it with our variables

[tsalo]

Yes, I believe that the json file is now generated by heudiconv, so we just need to add new variables and handle the merge.