pending_orphan_list and pending_orphan migrations and models

Part of Jira story: https://osraav.atlassian.net/browse/OSRA-70

Task: https://osraav.atlassian.net/browse/OSRA-166

Creating two new models:

- PendingOrphanList: to store an orphan list object while it is being validated and before the user decides to import its orphans (i.e. before it is stored in the "real" orphan_list table).
- PendingOrphan: to store a simplified orphan object (a hash of attributes with no associations) coming from an Excel file. This will be used to display extracted orphans on the orphan list validation results view and then converted to "real" orphan object once the user decides to import.
How does this work?