[ticket/12648] Improve tabs in ACP/UCP/MCP/Posting (fixes and cleanup)

https://tracker.phpbb.com/browse/PHPBB3-12648
https://tracker.phpbb.com/browse/PHPBB3-12443
https://tracker.phpbb.com/browse/PHPBB3-12605

PHPBB3-12648
Unrelated. Same for .bg3 below.