Bring several small improvements to WordPress RSS feeds

[jdevalk]

Context

• Brings some of the RSS feed enhancements we've long done on yoast.com to Yoast SEO.

Summary

This PR can be summarized in the following changelog entry:

• Adds canonical HTTP headers from RSS feeds to their parent URLs (for instance your homepage, or specific categories or tags), so the feeds themselves don't show up in search results.
• Makes sure the title separator chosen in Yoast SEO is used for RSS feed titles too.
• Makes sure the link tag in the RSS feeds' channel section links to the most specific URL possible (for instance the category or tag the RSS feed is for) instead of the homepage.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Make sure that feeds are enabled

• Repeat the checks below for all these types of feeds:

  • category feeds e.g. http:https://basic.wordpress.test/category/configurable-responsive-framework/feed/
  • tag feeds e.g. http:https://basic.wordpress.test/tag/assimilated-dedicated-flexibility/feed/
  • custom taxonomy feeds e.g. http:https://basic.wordpress.test/yoast-test-book-genre/crime/feed/
  • custom post type feeds e.g. http:https://basic.wordpress.test/my-books/feed/
  • post author feeds e.g. http:https://basic.wordpress.test/author/ansel73/feed/
  • post comment feed e.g. http:https://basic.wordpress.test/2022/06/adaptive-homogeneous-challenge/feed
  • for date archives feeds e.g. http:https://basic.wordpress.test/2022/feed/

• In the Network tab of your inspector, see that there is a Link HTTP header with rel="canonical" for every RSS feed, that has the URL of the page related to the feed (i.e. without /feed in the end)

• See that the <link> element in the <channel> section links to the same URL

• See that the separator used in the title is the one selected in Yoast SEO > Search Appearance

• Repeat the above check

  • for the home page [global comment feed] e.g. http:https://basic.wordpress.test/feed/
  • for search results feeds e.g. http:https://basic.wordpress.test/?s=and&feed=feed

• In the Network tab of your inspector, see that there is a Link HTTP header with rel="canonical" for every RSS feed, that has the URL of the home page

• See that the <link> element in the <channel> section links to the URL of the home page

• See that the separator used in the title is the one selected in Yoast SEO > Search Appearance

• If you test feeds that have multiple pages and add ?paged=2 to them, e.g. http:https://basic.wordpress.test/category/configurable-responsive-framework/feed/?paged=2, the URL in the <link> tag and in the Link header should be e.g. http:https://basic.wordpress.test/category/configurable-responsive-framework/page/2

When testing this on yoast.com, be sure to delete site/core-yoastcom/WordPress/Integration/YoastSEO/Feeds.php first.

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended
• I have written this PR in accordance with my team's definition of done.

Fixes PRODUCT-179

[enricobattocchi]

CR & test OK!