Loans migrator should be more robust

When migrating loans, the goal of the process should be to load every loan, only failing when an item or patron does not exist in the system or the item has another open loan. To this end, I propose adding the following logic to the loans_migrator task:
* Pull patrons from FOLIO, instead reading them from local files
* Pull items from FOLIO, instead of reading them from local files
* If an item has a status that is not loanable, change the status (eg. missing, etc., set a staff note and an item check-in note indicating the original status)
* If a patron does not have a barcode, fall back to the legacy patron ID (if present, similar to how proxy borrower support is implemented, remove it from the patron record when the loan is complete)
* If an item does not have a barcode, fall back to the item UUID (via legacyId) (add it as the barcode, and remove it when the loan is created)