[MM-23535] Add disable database search flag and return empty posts results if set (#14245)

* [MM-23535] Add disable database search flag and return empty posts results if set

* Add UpdateConfig function for the SearchStore and hook it into the app lifecycle

* Add the config listener in the server instance instead of using FakeApp

* Instantiate searchlayer as a pointer to avoid passing around copies of it

model/config.go

@@ -970,6 +970,7 @@ type SqlSettings struct {
 	Trace                       *bool    `restricted:"true"`
 	AtRestEncryptKey            *string  `restricted:"true"`
 	QueryTimeout                *int     `restricted:"true"`
+	DisableDatabaseSearch       *bool    `restricted:"true"`
 }
 
 func (s *SqlSettings) SetDefaults(isUpdate bool) {
@@ -1018,6 +1019,10 @@ func (s *SqlSettings) SetDefaults(isUpdate bool) {
 	if s.QueryTimeout == nil {
 		s.QueryTimeout = NewInt(30)
 	}
+
+	if s.DisableDatabaseSearch == nil {
+		s.DisableDatabaseSearch = NewBool(false)
+	}
 }
 
 type LogSettings struct {