buddy list: Extract user_search.js.

This was a bit more than moving code.  I extracted the
following things:

        $widget (and three helper methods)
        $input
        text()
        empty()
        expand_column
        close_widget
        activity.clear_highlight

There was a minor bug before this commit, where we were inconsistent
about trimming spaces.  The introduction of text() and empty() should
prevent bugs where users type the space bar into search.

tools/test-js-with-node

@@ -73,6 +73,7 @@ enforce_fully_covered = {
     'static/js/user_events.js',
     'static/js/user_groups.js',
     'static/js/user_pill.js',
+    'static/js/user_search.js',
     'static/js/util.js',
 }