Send canonical link header instead of noindex header

[rrit (Ayke)]

@Falco

This is rolling back the recently introduced site setting into what’s basically a noop (moving what we send as a head tag in a header, no semantic change).

I do not want this change as is.

The noindex change has been rolled back recently:

• Search engines now blocked from indexing non-canonical pages - #30 by sam
• FEATURE: enable canonical url indexing by SamSaffron · Pull Request #16196 · discourse/discourse · GitHub

May you have a new look at this PR:

FEATURE: Send a 'canonical' link header in non-canonical responses