Rename "Avatar" to "Profile Photo".

"Profile photo" is probably more easily understood by most people (and translators), and better describes what BuddyPress' member and group avatar features actually are; that we're asking for a person's photograph. Sometimes, avatar can mean "graphic" or "illustration".

This subtle change in phrasing also makes BuddyPress more consistent with the significant majority of other online platform services (Twitter, Facebook, etc).

Fixes #5758, props sooskriszta

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8677 cdf35c40-ae34-48e0-9cc9-0c9da1808c22