Add TargetPage model and mapper to notifications

This will be used for marking a notificaiton as read when
a user visits a target page. The new table should keep the
volume as low as possible for fast data loopup. records
should be removed from the table once it's marked as read.

Change-Id: I605cbc79adfc12d22bd889c5bb513d05c479fe6e
Merge "Add TargetPage model and mapper to notifications"