Does not display change-avatar user nav and screens if WordPress Show avatars setting is disabled

If WordPress discussion Avatar display setting is not activated, then displaying change-avatar user nav and screens is not necessary and might confuse the user as his uploaded avatar will not display.

Fixes #5346

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7747 cdf35c40-ae34-48e0-9cc9-0c9da1808c22