Support migrating data without users/groups

Problem Definition

Right now the JIRA Migration assistant only supports migrating projects together with users.
While this is a needed approach to import data with the correct ownership into Cloud this blocks test migrations where invalid users are present in the server userbase(users with a null email address for example). Those customers need to first clean up the server userbase before being able to use the assistant and test their migration.

Suggested Solution

Add an option to ignore some or all users so admins can test the migration assistant without needing to clean up any invalid accounts in the server userbase.

OR

Have JCMA figure out which users have no data at all, and which users have no access to Jira, and exclude those users from the set of users to migrate.