Allow bulk-nonymizing users

I really appreciate the anonymization-function introduce in Jira 8.7. But we have about 10 users per month to be anonymized. Each user takes about 3-5 minutes until done. We like to anonymize users in bulk.

We use a technical user for transferring ownership, so wo don't care about individual ownership transferring. But during the anonymization process, we like to now the anonymized user name once as described in JSDSERVER-6881.