Further preparation for Rails 6 (#15916)

* Use ActiveRecord::Result#to_ary instead of deprecated to_hash

They do the same thing, and to_hash has been removed from Rails 6.1

* Explicitly name polymorphic indexes to workaround a bug in Rails 6.1

cf. https://github.com/rails/rails/issues/41693

* Fix incorrect usage of “foreign_key” in migration script

* Use `ActiveModel::Errors#delete` instead of deprecated clear method

* Fix link headers tests on Rails 6.1

Rails 6.1 adds values to the Link header by default, thus it is not a
LinkHeader object anymore. Fix the test to parse the Link header instead
of assuming it is a LinkHeader.

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