Make organization short_names unique

The short_name field on the Organization model has been made unique.

This makes the model reflective of a reality that already exists on edX.org, and realizes an informal assumption that we make throughout edx-platform code.

For Open edX instances that enable FEATURES['ORGANIZATIONS_APP'], running this migration will fail if you have rows in the LMS/Studio organizations_organizationtable with identical values of short_name (noting that "identical" is case-insensitive for MySQL). If your instance has organizations with identical short_names, the easiest fix is to delete conflicting rows via Django admin (making sure that they have no inbound references from other model instances), and the re-attempting to run the migration. You probably would have wanted to do this anyway.