Safari enforces form-action against every hop in a form submission's
redirect chain. When a user signed in (with TOTP, or through a
skip_consent OIDC app), the chain /signin or /totp-verification ->
/oauth/authorize -> external client got blocked at the cross-origin
hop because form-action was 'self'. The existing dynamic CSP widening
in OidcController#authorize only ran when the consent page rendered,
so skip_consent and pre-consented flows had no widening at all.
Add allow_oauth_redirect_in_csp on the sign-in and TOTP pages, which
pulls the OAuth redirect_uri out of session[:return_to_after_authenticating]
and appends its host to form-action for the rendered page.
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