Welcome Screen: update "new install" messaging.

Resembling the design of the top part of WordPress' dashboard, the first column shows the list and links to Settings > BuddyPress panels, the second column lists the BP panels in wp-admin, and the third column shows the links to the support forums and codex.

Props mercime, fixes #5840

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8980 cdf35c40-ae34-48e0-9cc9-0c9da1808c22