[ticket/12408] Add quick setting for "default guest style" to ACP

PHPBB3-12408
[ticket/12408] Simplified queries

PHPBB3-12408
[ticket/12408] Code cleanup

Rebased to see if travis will finally play along

PHPBB3-12408
[ticket/12408] Public functions and updated doc block

PHPBB3-12408
[ticket/12408] Changed lang vars, added fieldset and removed "default"

PHPBB3-12408
[ticket/12408] Clarify the behavior of "override style"

PHPBB3-12408
[ticket/12408] Fix white-space sniffer

PHPBB3-12408
Merge pull request #2307 from PayBas/ticket/12408

[ticket/12408] Add quick setting for "default guest style" to ACP