[ticket/13044] Correct Expires headers

PHPBB3-13044
[ticket/13044] move GMT string out of gmdate()

PHPBB3-13044
[ticket/13044] \G\M\T to GMT in whole project

Note that there are more uses of "\G\M\T" in vendor files.

PHPBB3-13044
[ticket/13044] Move GMT out of gmdate()

PHPBB3-13044
[ticket/13044] Move GMT out of gmdate in functions_acp

PHPBB3-13044
Merge pull request #2962 from Senky/ticket/13044

[ticket/13044] Expires header violates RFC 2616

* Senky/ticket/13044:
  [ticket/13044] Move GMT out of gmdate in functions_acp
  [ticket/13044] Move GMT out of gmdate()
  [ticket/13044] \G\M\T to GMT in whole project
  [ticket/13044] move GMT string out of gmdate()
  [ticket/13044] Correct Expires headers