Activity comment query still joins against users table

[boonebgorges]

In [7765], primary activity queries were split into ID lookups and data lookups. The same should happen in activity comment queries (get_activity_comments())

[boonebgorges]

In 7776:

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

[boonebgorges]

In 7777:

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