Exclude Rails database schema from linting

The schema is automatically generated, so RuboCop's recommendations are
non-actionable; ignore it as in rubocop-hq/rubocop#752.

.rubocop.yml

@@ -6,6 +6,8 @@ AllCops:
   UseCache: true
   CacheRootDirectory: tmp/rubocop_cache_rails_dir
   MaxFilesInCache: 4000
+  Exclude:
+    - db/schema.rb
 
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