MagicWordFactory to replace MagicWord static members/methods

Static members of MagicWord have been removed.

Static methods are soft-deprecated and forward to the factory. They
will be hard-deprecated when all callers are removed from core.

MagicWord::clearCache() has been removed. Instead, call
resetServiceForTesting( 'MagicWordFactory' ) on your MediaWikiServices
object.

Change-Id: Ie061fe90f9b9eca0cbf7e8199d9ca325c464867a
Bug: T200247