[ticket/12858] Remove hardcoded language entries from timezone selects

PHPBB3-12858
[ticket/12858] Fix timezone display on acp users page

PHPBB3-12858
[ticket/12858] Fix timezone select display in subsilver2

PHPBB3-12858
[ticket/12858] Rename tz_ prefixed block variables to timezone_ prefix

PHPBB3-12858