Batch user create

PLEASE DO NOT MERGE until signed off by @mnowster for assurance concerns around processing of file uploads.

From the user's perspective, this adds the ability for admins and superadmins to create users in batch from a CSV. Users in a given batch are assigned the same permissions. Preexisting users are skipped.

From an architecture perspective, we are doing two new things:
1. (actively) accepting and processing file uploads
2. adding background workers with DelayedJob

There will be a (fairly trivial) Pull Request to add the puppet configuration for running the background worker(s).