Add and apply Rails/WhereNot rubocop rule

We simplify the code a little bit, and make it more consistent since we
were already using `where.not` in most places.

app/models/milestone.rb

@@ -15,7 +15,7 @@ class Milestone < ApplicationRecord
 
   scope :order_by_publication_date, -> { order(publication_date: :asc, created_at: :asc) }
   scope :published,                 -> { where("publication_date <= ?", Date.current.end_of_day) }
-  scope :with_status,               -> { where("status_id IS NOT NULL") }
+  scope :with_status,               -> { where.not(status_id: nil) }
 
   def self.title_max_length
     80