Added a queue for convert.py to work in a background process

I've added a "queue" collection in the Mongo database.

Added some CRUD method in datafetch.py.

Modified main.py so when we upload a movie the conversion is not started, but instead a new queue item is created.

Changed the convert.py to be ready to be called from a cron job. Basic flow:

  - Check for available queue item (where process_started = None)
  - Each item is tagged process_started = Now
  - For each item, the same conversion webm and mp4 is the same
  - TODO: Between the webm and mp4 there's a call to datafetch.update_queue to change the progress.

For now we need to manually run convert.py after an upload (i.e. the cron job was not added in the init.sh file yet).