We should add a collection export endpoint with a UI button for saving a whole collection to a .eln file: https://github.com/TheELNConsortium/TheELNFileFormat

Initially, just folder per sample, zip up all attached files and metadata and let the user download

This might be too big to do in one process/request, so may need to think about endpoint that works like:

1. /export/ -> API returns "ok i'll do it" with a link like /elnexport/asdfdsafa
2. API triggers background process to create .eln file (Celery, APScheduler, ...) -- probably just our mongo database with a new collection tasks
3. UI has to ping that link /elnexport/asdfdsafa and API will respond ready or not-ready with another link for download