bp-legacy: Add JS to toggle the "Blog Details" block on registration page.

When theme compatibility was introduced in BuddyPress 1.7, the javascript
to toggle the site registration form fields was not ported over from
bp-default.  This commit brings back this functionality to bp-legacy.

Fixes #5846.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9250 cdf35c40-ae34-48e0-9cc9-0c9da1808c22