Studio Import does not update users and groups correctly if there is existing data on the instance

Users and groups are not correctly imported by the Studio Importer if there is existing data on the instance.

Other entities are handled via JIRA's normal restore functionality, which blows everything away before importing the new data. However, with users and groups, there is OnDemand-specific logic to synchronise the imported data with the Crowd directory. This logic is incomplete, in that it doesn't update attributes of existing users, does not remove users, and only creates new group membership entries (does not remove any).

This impacts JIRA Enterprise hosted to OnDemand migrations. Details here: https://sdog.jira.com/browse/JSTDEV-1935?focusedCommentId=20853&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-20853

The solution is to delete all users (except the admin performing the import and sysadmin) and all groups (except the initial set of groups expected on OnDemand) at the beginning of the Studio import process.