Welcome screen: tidy up for 2.1 by removing 2.0-specific media

See #5805

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8774 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Welcome screen: add nav bar and logo to Credits tab

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8775 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Welcome screen: update credits list for 2.1 so far

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8776 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Welcome screen: add new credits section for external libraries

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8777 cdf35c40-ae34-48e0-9cc9-0c9da1808c22