* DEV: Remove with_deleted workarounds for old Rails version
These workarounds using private APIs are no longer required in the latest version of Rails. The referenced issue (https://github.com/rails/rails/issues/4306) was closed in 2013. The acts_as_paranoid workaround which this was based on was removed for rails > 5.
Switching to using a scope also allows us to use it within a `belongs_to` relation (e.g. in the Poll model). This avoids issues which can be caused by unscoping all `where` clauses.
Predicates are not necessarily strings, so calling `.join(" AND ")` can sometimes cause weird errors. If we use `WhereClause#ast`, and then `.to_sql` we achieve the same thing with fully public APIs, and it will work successfully for all predicates.
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