Template functions to get activity item's comments' author IDs

[DJPaul]

In the activity loop, it's not easy to get data about the authors of all activity comments belonging to a single activity item. You either have to write code and loop through the global, or perform two activity loops; the first to get some piece of data, then the second to render the template.

Storing the data can be tricky, too, as the activity item and activity comment templates are split across several files. Theme developers shouldn't have to expect to iterate through our globals and internal data structure.

[DJPaul]

An example use case is where you want to show a list of all users participating in a discussion on a particular activity, above or before the activity comments themselves.

[djpaul]

(In [6501]) Add template functions to get an activity item's comments' author IDs

• bp_activity_get_comments_user_ids() returns the user IDs of everyone who's written an activity comment on the current activity item.
• bp_activity_comments_user_avatars() uses the above to output the users' avatars, linked to their profile page, into <LI> tags.
• Fixes #4655

An example use case is where you want to show a list of all users participating in a discussion on a particular activity, above or before the activity comments themselves.