Clean up subscriptions when user is deleted.

app/models/user.rb

@@ -110,6 +110,7 @@ class User < ActiveRecord::Base
   has_many :notes,                    dependent: :destroy, foreign_key: :author_id
   has_many :merge_requests,           dependent: :destroy, foreign_key: :author_id
   has_many :events,                   dependent: :destroy, foreign_key: :author_id,   class_name: "Event"
+  has_many :subscriptions,            dependent: :destroy
   has_many :recent_events, -> { order "id DESC" }, foreign_key: :author_id,   class_name: "Event"
   has_many :assigned_issues,          dependent: :destroy, foreign_key: :assignee_id, class_name: "Issue"
   has_many :assigned_merge_requests,  dependent: :destroy, foreign_key: :assignee_id, class_name: "MergeRequest"