[B] Improve messages, show player count - Fixes BUKKIT-5427

**The Issue:**
The white-list command lacks a few features for server management, and can also cause some confusion. I believe the changes below would help solve these issues as explained below.

**Justification for this PR:**
This PR is aimed at helping ease of use with the white-list command, and removing potentially confusing components.

There is two main modifications that this PR presents:

1) The listing of white-listed players
Currently when viewing the white-list, if no players have been added the list will be empty. This is not clean, and has the possibility for confusion as it gives the impression there should be white-listed players. Now, You would receive a clean straightforward message saying no players have been white-listed, or a list of players who have been white-listed (and also a handy count).

2) Alerting the player when the specified target is, or is not on the white-list when trying to add/remove the target from the white-list
Mainly to prevent confusion, The current commands for adding and removing players, should tell the player if the target is, or is not already white-listed.

**Testing Results and Materials:**
The changes were compiled locally, then tested on a local server with two players. The tests confirm the changes work, and the command has intended use.

**JIRA Ticket:**
https://bukkit.atlassian.net/browse/BUKKIT-5427