Allow users to opt out of the GDPR changes to username

As mentioned in the https://developer.atlassian.com/cloud/bitbucket/bitbucket-api-changes-gdpr/, the username field has been replaced by the account_id field.

It can be tricky to look up user accounts as a result of this, especially from the perspective of a team admin who needs to manage all users within the team/organization across accounts.

It would be great if users are given the choice on whether to adhere or to opt out of the new changes via a toggleable configuration.