MM-24530-Add support for search terms with underscore using postgresql engine (#16618)

* add support for search terms with underscore using postgresql

* fix failing tests

Initially issue fix applied to all search terms instead of the ones that contains _

* direct cast tsquery

* fix test issues

* refactor underscored quoted term search

* support search term case-insensitive

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/searchtest/post_layer.go

@@ -215,7 +215,7 @@ var searchPostStoreTests = []searchTest{
 	{
 		Name: "Should be able to search terms with underscores",
 		Fn:   testSearchTermsWithUnderscores,
-		Tags: []string{EngineMySql, EngineElasticSearch},
+		Tags: []string{EngineAll},
 	},
 	{
 		Name: "Should be able to search posts made by bot accounts",