[ticket/12963] Revert back to "migrations" folder name for extensions

The issues that can be created with the name change are just too much

PHPBB3-12963
[ticket/12963] Remove old migration/ folder from tests

PHPBB3-12963
[ticket/12963] Always use core_path and core_directory()

PHPBB3-12963
Merge pull request #2967 from nickvergessen/ticket/12963

[ticket/12963] Revert back to "migrations" folder name for extensions

* nickvergessen/ticket/12963:
  [ticket/12963] Always use core_path and core_directory()
  [ticket/12963] Remove old migration/ folder from tests
  [ticket/12963] Revert back to "migrations" folder name for extensions
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12963] Always use core_path and core_directory()
  [ticket/12963] Remove old migration/ folder from tests
  [ticket/12963] Revert back to "migrations" folder name for extensions
Merge pull request #2875 from PayBas/ticket/12961

[ticket/12961] Add link in anti-spam ACP page which links directly to captchas in titania

* PayBas/ticket/12961:
  [ticket/12961] Shorten ext-db link text
  [ticket/12961] Fix missing phpBB.com
  [ticket/12961] Remove target="_blank" and switch to <strong>
  [ticket/12961] Add additional link to the KB
  [ticket/12961] Move link to language file
  [ticket/12961] Refactor lang entry
  [ticket/12961] Change url to anti-spam-ext
  [ticket/12961] Change url to https://www.phpbb.com/go/anti-spam/
  [ticket/12961] Add link in anti-spam ACP page to titania
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12961] Shorten ext-db link text
  [ticket/12961] Fix missing phpBB.com
  [ticket/12961] Remove target="_blank" and switch to <strong>
  [ticket/12961] Add additional link to the KB
  [ticket/12961] Move link to language file
  [ticket/12961] Refactor lang entry
  [ticket/12961] Change url to anti-spam-ext
  [ticket/12961] Change url to https://www.phpbb.com/go/anti-spam/
  [ticket/12961] Add link in anti-spam ACP page to titania