Change canonical link-tag for posts Ember-url to use new canonical url

[rrit (Ayke)]

The following PR introduced new canonical urls for posts. And I really appreciate this!

• FEATURE: use canonical links in posts.rss feed by SamSaffron · Pull Request #16190 · discourse/discourse · GitHub

Old links [= Ember-url] were of the form:

https://DOMAIN/t/SLUG/43/21

New links [= canonical url] are of the form

https://DOMAIN/t/SLUG/43?page=2#post_21

In crawler-view check the canonical link-tag for a posts Ember-url like:
https://meta.discourse.org/t/feedback-on-new-hamburger-and-user-menus/32519/123

<link rel="canonical" href="https://DOMAIN/t/SLUG/32519?page=6">

This should be changed to contain …#post_123:

<link rel="canonical" href="https://DOMAIN/t/SLUG/32519?page=6#post_123">

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Ressources on anchors in canonical-tags:

• https://moz.com/community/q/topic/59435/can-i-set-a-canonical-tag-to-an-anchor-link

  […] use rel=canonical to point them to their relevant sections [= anchors] […]

  For anyone who may care - this appears to have worked out really well. So far way better than expected. […]

• https://moz.com/blog/using-canonical-tag-to-get-more-than-one-anchor-text-value-11283 (2010)