[ticket/12858] Generate timezone selects with template loop

PHPBB3-12858
[ticket/12858] Use UTC for time info in footer and hide offset if it's 0

PHPBB3-12858
[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
[ticket/12858] Directly return and use timezone selects

PHPBB3-12858
[ticket/12858] Properly format UTC offset in footer

PHPBB3-12858
Merge pull request #2844 from marc1706/ticket/12858

[ticket/12858] Remove hard-coded GMT from timezone drop-down and rename to UTC