Why:
* A hook is needed which is called when User::spreadAnyEditBlock
is called, so that extensions which provide alternative blocking
mechanisims (such as the GlobalBlocking extension) can spread
their blocks when local blocks are spread.
What:
* Add SpreadAnyEditBlockHook which is called from User
::spreadAnyEditBlock when it is called except when the user is
not registered.
** The hook is called even if the user is not locally blocked
* The return value of User::spreadAnyEditBlock is modified to
return true if either a local block or alternative blocking
mechanism spread blocks.
* Update UserTest to test this new behaviour.
Bug: T374857
Change-Id: Id302a6362d6177c89da9cdf4e677b3822ecb85f1
6547b639d8