Update Rubocop preferences so Hound can complain about:
Linting rules on ERB or HTML files (erb-lint), which covers Ruby/Rails code in the templates. erb-lint can also nag about CSS classes we no longer want to keep using.
rubocop-rails and erb-lint need to be added at the same time for compatibility reasons: https://github.com/Shopify/erb-lint/issues/130.
How to test: none, it's just Hound.