Make sure xprofile_format_profile_field() returns a well formatted value in case of a datebox field.

This function is now using the {{{display_filter()}}} filters as defined by the corresponding {{{BP_XProfile_Field_Type}}} classes.

Props wbajzek, shanebp.

Fixes #4365

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8689 cdf35c40-ae34-48e0-9cc9-0c9da1808c22