Multi file upload

This is a pull request into sha256-custom branch.
So if you think that debugging prints are useful to keep in, then let's decide on a pattern to do that. I would suggest to use `Log.debug` from `logger` package and see how we could through Meteor settings enable debug logging for non-production runs. We are already using [settings](https://github.com/peerlibrary/peerlibrary/wiki/Settings#fields) to configure some debugging aspects and we could add additional field, like `logging`. So if `public.debug.logging` is set to true or if production is not set to true, we output debugging calls.