Enqueue BuddyPress JS scripts for bp-default.

When we removed the previously, bundled inline scripts from bp-default's
global.js in BuddyPress 2.1, we forgot to enqueue these new scripts into
bp-default.

This commit enqueues the various JS scripts, which fixes issues with
javascript breaking in bp-default.

See https://buddypress.trac.wordpress.org/ticket/5889