"Read More" AJAX duplicates across nested activity comments

[r-a-y]

Example:
​http://testbp.org/members/micheal/#activity-105105

Click on the "Read More" link in the activity comments.

The AJAX response duplicates for the child comment as well.

Looking into it.

[r-a-y]

Because nested activity comments share the same parent, the AJAX response was duplicating under activity comment childs.

Attached patch adds the ":first" selector to the Read More AJAX response, thus only expanding the AJAX response to the first match.

[boonebgorges]

(In [4790]) Ensure that only the intended activity comment is expanded via AJAX when clicking Read More. Fixes #3366. Props r-a-y