Use span tags instead of abbr tags in the footer

Replace abbr tags with span tags in places where they are not really used in a correct context, to eliminate the new underline effect caused by our updated abbr tag css.

1. navbar footer time zone info
2. debug footer info (time and memory usages)

See additional discussion here: https://tracker.phpbb.com/browse/PHPBB3-14103
regarding maintaining the original look and behavior of Prosilver (which was lost by updating abbr tags)