Deduplicate users in the userList macro

[slauriere]

Users should be deduplicated when present both as user and member of a listed group, and in any occasion where users can be duplicated.

Steps to reproduce

Create user list with users="XWiki.Admin" and groups="XWiki.AdminGroup"

Actual result

The user list displays XWiki.Admin twice instead of once

[raphj]

Fixed.

The fix is boring and unsurprising: I changed the ArrayList usage to a HashSet.