Address missing Fitbit files

For some participants, not all Fitbit files were retrieved - this could be due to the chain of Promises/fetches used to get the files (one failed retrieval might be crashing the rest), or it's an issue of slow internet connections, in which closing the window too soon stops the retrieval of files. First step would probably be to implement some kind of page timer that will wait a "safe" amount of time for all files to be retrieved before allowing the window to be closed (or better yet, automatically close the popup authorization window once all files are retrieved)