[Xon] Search Improvements 2.17.0

Enhanced specialized search with additional debug capabilities
Update specialized search to support SearchOrder type
Fix XF2.1 when using specialized search functionality (aka fix tag/user autocomplete with XF2.1)

Improve 3rd party support for specialized search index support

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

Compatibility fix for Username auto-complete using XFES and Tag auto-complete using XFES add-ons

Fix min/max range searches (ie word-count) where flipped when using MySQL full-text search

Fix guest user searches could be sent into a redirect loop depending on search criteria

Fix XF2.3.0 infinite search redirect for guests

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