Add-ons Search Improvements 2.17.1

• Add additional hookpoints for customizing "specialized search indexer" functionality
• On search results page, when searching a conversation(XF2.2)/direct message(XF2.3) display "with conversation/direct message"

• Search extensions API changes for better XF compatibility, required for Conversation Improvements/Essentials
  • XF2.3 compatibility fix for custom "SearchOrder" class
  • Simplify asSqlConstraint code (expected to always return an array)
  • AbstractConstraint::setMatchType needs to normalize to an int for MetadataConstraint::matchType

• Update installer to verify ElasticSearch index does not require rebuilding when this add-on is installed/updated/rebuilt
• Return 400 status code instead of 200 when the search query is wrong