activity.js: Extract get_filtered_and_sorted_user_ids().

This function was extracted from build_user_sidebar().  We
also slightly streamlined it to not unnecessarily call
filter() when the filter text was blank. This extraction
also eliminated the need for us to have the two-line
filter_and_sort() function.

Also, we get to 100% coverage in this commit.

tools/test-js-with-node

@@ -21,8 +21,7 @@ USAGE = '''
     '''
 
 enforce_fully_covered = {
-    # activity.js Removed while we debug some coverage issues
-    # 'static/js/activity.js',
+    'static/js/activity.js',
     'static/js/alert_words.js',
     'static/js/bot_data.js',
     'static/js/channel.js',