Improve JS for "Select" dropdown on messages screens

- Use jQuery delegation, to ensure that items added to the DOM dynamically (as
  in an AJAX search query) are listening for clicks
- Simplify logic

See #5070

Props imath

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7411 cdf35c40-ae34-48e0-9cc9-0c9da1808c22