Merge pull request #2932 from Nicofuma/ticket/12963

[ticket/12963] Load extensions migrations from /migration
Merge branch 'develop-ascraeus' into develop
Merge pull request #2922 from Nicofuma/ticket/13036

[ticket/13036] Make the reference type configurable when generating a route
Merge branch 'develop-ascraeus' into develop
[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/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/13044] Move GMT out of gmdate()

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

PHPBB3-13044
[ticket/12858] Directly return and use timezone selects

PHPBB3-12858
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
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [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
Merge pull request #2951 from Nicofuma/ticket/13064

[ticket/13064] Validate the migrations provided to migrator::set_migrations()

* Nicofuma/ticket/13064:
  [ticket/13064] Validate the migrations provided to migrator::set_migrations()
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/13064] Validate the migrations provided to migrator::set_migrations()