As suggested to me by email: very active members of a wiki community can end up recei...

As suggested to me by email: very active members of a wiki community can end up receiving hundreds of emails via Special:EmailUser, the majority of which will end up with the boilerplate "{{SITENAME}} email" subject, making them hard to distinguish.  Pass the name of the user sending the email to the message, so that it can be at least a little bit more unique.