Disable activity reply threading if WP comment threading is disabled

In r8201, BP adopted WP's thread_comments_depth setting for its own activity
reply threading. See #2768. However, the thread_comments toggle was not
respected. The current changeset adds support for thread_comments, so that
disabling WP comment threading will also enable nested activity comments
(though the top level of comments will still be allowed).

Fixes #5562

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8402 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Minify some admin images

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8403 cdf35c40-ae34-48e0-9cc9-0c9da1808c22