Add ScopedCallback::newScopedIgnoreUserAbort helper function

It sets ignore_user_abort( true ) for non-CLI requests until the callback
is triggered for scenarios where you don't want to be interrupted.

This was previously implemented in 3 different places in MediaWiki:

• FileBackend::getScopedPHPBehaviorForOps()
• LBFactory::getScopedPHPBehaviorForCommit()
• LoadBalancer::getScopedPHPBehaviorForCommit()

Bug: T184044
Change-Id: Ib6e307d76f9328c11d2bed3617552ddc78124665