Add bulk sent mailing import helpers for CiviMail

CiviMailBulkStore::addSentBulk will add bulk mailing records
for a sent mailing.  It does not yet add bounce records.

Change-Id: I43dcd57d9916530a6e9f430ea0eec2079fd51838
Put all CiviMail stuff into wmf_communication namespace

Change-Id: Ife96268a776892fa66f5291ff7f82b0a62921c70