Add-ons Search Improvements 2.17.1

Thanks @NamePros for sponsoring this update

• Fix "before" search result term would print an integer instead of a human readable yyyy-mm-dd formatted string
• Change 'no results found' handling to display search terms on the relevent page
  • If using ElasticSearch Essentials, recommend updating to v3.13.0+ as this change may impact some options which display on no results found.
• Fixes for search term on results page
  • Add missing "With X thread" search term
  • Fix "Users" search term could fail to render the list of usernames
  • Add the 'content type' search term

• Remove usage of utf8_* function(s), and use native php multi-byte functions instead.
• Fix HTML markup error in svPushViewOtherCheckIntoXFES option description

• Fix bug where 'weight by content type' feature didn't work as expected in general search
• Thanks to @NamePros for sponsoring this update.
• Display various search term constraints on the search results form.
  For developers to implement support in 3rd party add-ons:
  • Each search constraint needs a svSearchConstraint. prefixed phrase.
    Arrays are mapped to phrases by adding a _ for each sub-array/key as such; c[warning][points][lower] => svSearchConstraint.warning_points_lower
  • Each search order needs a svSearchOrder. prefixed phrase.
  • Extend XF\Entity\Search::getSpecializedSearchConstraintPhrase(string $key, $value) to provide custom phrase handling (ie node names)
  • Extend XF\Entity\Search::formatConstraintValue(string $key, $value) to provide custom formatting.
  • Extend XF\Entity\Search::setupConstraintFields to populate $svDateConstraint/$svUserConstraint/$svIgnoreConstraint properties which control formatting
• Use the debug option "List all unphrased search constraints" which will dump unmapped contraints to search results page.