Comment group-based JQL predicates need to search on userKey, not userName

The JQL functions lastComment and commented currently search the Lucene index by means of a username. For many JIRA instances, where username and user key are identical, this is fine.

However, for instances where the user key differs from the username, this can return incorrect results for role and group-based queries.

This should be fixable using the getUsersInGroup method, which would also support nested groups.