After successful avatar crop, redirect to user domain

Previously, we'd redirected back to the Change Avatar page. But on a
successful crop, this is unneeded. Instead, we send the user back to
his profile, with a success message.

Fixes #5150

Props henrywright

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7359 cdf35c40-ae34-48e0-9cc9-0c9da1808c22