Set a switch to retain ye olde `bp-default` theme if it's been activated previously. Fixes issues with theme switching, and `bp-default` disappearing. Props boonebgorges. Fixes #5325. (trunk)

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7798 cdf35c40-ae34-48e0-9cc9-0c9da1808c22