Log skips

A target may be skipped for multiple reasons:

* The target is not in $wgNamespacesToPostIn
* The target is in the opt-out category
* The delivery is in NS_USER or NS_USER_TALK and the
  associated account does not exist

These differ from failures because we are intentionally
not sending a message to the target. Logging is useful
so the sender is not confused as to why a certain
recipient did not get the message.

Change-Id: Id638a8a3790ceece8ec82fc03d996f60c5d2e5f7
Merge "Log skips"