Introduce xprofile_get_field_visibility_level()

This is a simple function for grabbing the visibility level attached to a given
field by a specific user, sensitive to admin-provided defaults and override
settings.

See #4636

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7836 cdf35c40-ae34-48e0-9cc9-0c9da1808c22