Change show/hide storage mechanism for show_welcome_panel. 0 = hide, 1 = show,...

Change show/hide storage mechanism for show_welcome_panel. 0 = hide, 1 = show, 2 = show if the multisite owner. Make welcome panel dismissable without JS (openable too via the URL). fixes #19338, fixes #19127.

git-svn-id: https://core.svn.wordpress.org/trunk@19418 1a063a9b-81f0-0310-95a4-ce76da25c4cd