Disallow Duplicate Barcode in Users

Steps to repro:
1. log into folio
2. Go to users and copy the barcode for an existing user
3. Create a new user and give them the same barcode

Expected behavior: When non-null user barcode is entered AND barcode is not unique:

• Validation message should display reading "This barcode has already been taken"
• Save should be prevented
  Note: There can be any number of records with NULL barcode for patrons without a (valid) library card.

Actual: No validation and record is saved