Replace "this wiki" with SITENAME in "searchmenu-exists"

Then read my message again please. There are at least few wikis that changed this message only to use their wiki names instead of generic "this wiki" text. Saving the time of admins on another ones is my key point here but if you want to be against my idea just for being against, feel free.

The fact that some wikis do it doesn't tell us why they did it, nor that it would be appropriate for other wikis.