Clarify emailuserfooter mw-message

Clarify emailuserfooter mw-message

Recently we modified this message to add a privacy disclaimer which is
automatically added to outbound email sent using the Special:EmailUser
function on MediaWiki.

The disclaimer, notwithstanding, it only shown for the receiver of the
message so we should clarify that their email address will be disclosed to
the original sender only if they decide to reply to the email.

This patch therefore adds a "If you reply"-like wording to the
emailuserfooter message clearly stating that only if the user decides to
reply the message, their email address will be indeed disclosed to that
original sender.

Change-Id: Ie2ccf26b078e768f615eea8c8a83d92ff83abce0