[Xon] Search Improvements 2.16.1

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

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

[*]Add "Push thread prefix text into search" option (default true)
[*]Supports ElasticSearch Essential's autocomplete
[*]Requires manual rebuild of threads on changing this option

Fix guests searching by user would restart the search query the first time they tried to visit the a different page of the search results