Race condition in realm creation

[alexmv]

It's possible for two realm creations with the same name to race, which triggers a 500 and raises an exception:

duplicate key value violates unique constraint "zerver_realm_subdomain_key"
DETAIL:  Key (string_id)=(duplicate-realm-name) already exists.

This is raised by realm.save() in do_create_realm, which is (correctly) inside a transaction.atomic() so we don't get inconsistent state in the database. The only problem is the potential user experience; accounts_register in zerver/views/registration.py should catch the IntegrityError that is so raised, and return a 400, not a 500.

[zulipbot]

Hello @zulip/server-production members, this issue was labeled with the "area: production" label, so you may want to check it out!

[cognusion]

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[zulipbot]

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[cognusion]

PR #22006 has been open for this issue. :patientlywaiting: on that 😄