Added VERP functionality hook to core

* By default, wiki should keep the envelope sender as $wgPasswordSender
* Requires BounceHandler extension. Added hook to core
* Alters the return path, to bounce-{key}@domain
* The envelope sender is set using the 5th param in mail()

Bug: 46640
Needed By: I9463ae33ae327405725ea9693d45ad61b8ecf798
Change-Id: Ic5c1231611a5c4984ddf81fd716e6f3a3e82fd57