Activity template function to get parent activity's user ID

[DJPaul]

bp_get_activity_parent_content() exists and returns the content of the parent activity item. This is used for the "in reply to" link, when an activity comment reply appears in a person's activity stream. I'd like to add a function which returns the activity parent's user ID, so templates can display information about the user that the comment was in reply to.

Patch attached.

[boonebgorges]

This looks good and useful to me.

[djpaul]

(In [6523]) Add new Activity template loop function to get the parent activity's user ID. Fixes #4663