Remove project notification settings associated with deleted projects

On GitLab.com, there are 1560 project notification settings with no valid project all created on 2016-04-15 10:38:53 for some reason. This migration should purge
those entries.
    
b8f38437 should prevent this issue from occurring in the first place.
    
Closes gitlab-com/support-forum#678


See merge request !4311