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.

db/dev_seeds/flags.rb

@@ -1,19 +1,19 @@
 section "Flagging Debates & Comments" do
   40.times do
     debate = Debate.all.sample
-    flagger = User.where(["users.id <> ?", debate.author_id]).all.sample
+    flagger = User.where.not(id: debate.author_id).all.sample
     Flag.flag(flagger, debate)
   end
 
   40.times do
     comment = Comment.all.sample
-    flagger = User.where(["users.id <> ?", comment.user_id]).all.sample
+    flagger = User.where.not(id: comment.user_id).all.sample
     Flag.flag(flagger, comment)
   end
 
   40.times do
     proposal = Proposal.all.sample
-    flagger = User.where(["users.id <> ?", proposal.author_id]).all.sample
+    flagger = User.where.not(id: proposal.author_id).all.sample
     Flag.flag(flagger, proposal)
   end
 end