Enable per_form_csrf_tokens
Enable per_form_csrf_tokens

Rails 5 introduced[1] the option to generate a different CSRF token
for each form. This mitigates a certain type of same-origin
content-injection attack[2]. Enabling the option seems sensible and
doesn't require any other changes to be made.

[1] https://github.com/rails/rails/commit/3e98819e2
[2] https://github.com/rails/rails/pull/22275

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