welcome: rename {{welcome-anon-vandal}} to {{welcome-anon-unconstructive}}

Per request at WT:TW.
welcome: place {{welcome-screen}} with static=true

I probably shouldn't make this kind of unilateral change, but I really
do feel sorry for new users who get this amateurish, distracting,
annoying bouncy WP logo slapped on their user talk page - some welcome!