Add-ons Search Improvements 2.17.0

• 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

• Restore XF2.1 support
• XF2.3 support
• Fix "[E_DEPRECATED] Use of "parent" in callables is deprecated" on search