Introduce BP_Activity_Activity::append_user_fullname()

Previously, the appending of xprofile fullnames was done inline in the get()
method. This changeset breaks it into a standalone method, so it can be used
elsewhere in the class.

See #5371

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7776 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Split activity comments query into searate ID query

This eliminates joins against the global user table, and also centralizes
caching for individual activity items.

Fixes #5371

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7777 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Static method should be marked static

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7778 cdf35c40-ae34-48e0-9cc9-0c9da1808c22