Lint for unused rails helpers

While doing some refactoring, it was discovered that we had some rails helpers weight_dropdown_tag and weight_dropdown_label which were not utilized anywhere. These also had some associated frontend code. (see !61492 (merged))

These went unnoticed for about three years, and I'm sure there may be other instances of rails helpers which are unused in our codebase. We probably ought to find a way to programmatically detect when rails helpers or rails partials are orphaned.

/cc @leipert