LPS-47546 Private Messaging Portlet does not use the search index while searching users as part of autocomplete operation

Hey Hugo,

For this one, I had a couple of concerns but didn't know how to get around them:

1. When searching by the index, I couldn't use the UserFirstNameComparator anymore, so instead of ordering users by firstName, then middleName, and then lastName, it is by firstName and when they were added. However, the Sort only takes in one fieldName parameter so I don't see a way around this.

2. Searching for users via the index requires that the Portal Property "users.indexer.enabled" is set to true. Since I can't check Portal Properties from the plugin, I decided to leave this Portlet Property set to false and add in a warning comment to it.

If you know of a way around either of those or see any other issues, please let me know. Thanks.