Fixes #1478.

There were 3 variants on how to proceed:

1. ignore multiple has_paper_trail calls - this can be confusing, for example

class Parent < ApplicationRecord
  has_paper_trail on: [:create]
end

class Child < Parent
  has_paper_trail on: [:update]
end

For which events we should create version records? 🤷 Same without STI (just when we include some module and have the has_paper_trail called on the class) or just has_paper_trail defined on the ApplicationRecord and then in the concrete class too.

2. detect if we use STI and do something smart about it (as suggested in the issue) - this won't work, because when we detect that we use STI, the callbacks are already defined on the parent model and we can't "undefine" them
3. raise if we have multiple calls - implemented in this PR. Looks the most sane to me, but this is a breaking change