Minor improvements on command usage error messages

Fixes a bug where players were unable to reset their passwords

This new change introduces:

• Fixes a bug where specific players could not be seen by others (players are not hidden before authentication anymore)
• Prevents different players from using the same e-mail address

This update does not allow the player to issue any server command (except the ones used to authenticate and register) without being authenticated first

Adds email extension validation if provided and allows HTML/CSS styling for email template