lint: add a Rubocop rule to forbid the use of unscoped

`unscoped` often has an effect broader than intended. For instance:

```
user.dossiers.unscoped.destroy_all
```

will not destroy all the user's dossier, including the hidden ones,
but **all the dossiers of all users**.

Instead, narrower unscoping should be used:

```
user.dossiers.unscope(where: :hidden).destroy_all
```

(Or even better, use the new `Dossier.with_hidden` scope).

.rubocop.yml

@@ -1,5 +1,6 @@
 require:
   - rubocop/rspec/focused
+  - ./lib/cops/unscoped.rb
 
 AllCops:
   Exclude:

lib/cops/unscoped.rb

@@ -0,0 +1,18 @@
+module RuboCop
+  module Cop
+    module DS
+      class Unscoped < Cop
+        MSG = "Avoid using `unscoped`. Instead unscope specific clauses by using `unscope(where: :attribute)`."
+
+        def_node_matcher :unscoped?, <<-END
+          (send _ :unscoped)
+        END
+
+        def on_send(node)
+          return unless unscoped?(node)
+          add_offense(node)
+        end
+      end
+    end
+  end
+end