[MM-20979] Add first implementation of the Bleve search engine (#14562)

* [MM-20979] Add first implementation of the Bleve search engine

* Fix i18n

* Migrate searchengine utils tests

* Fix linter

* Don't add allTermsQ if both termQueries and notTermQueries are empty

* Fix test that should work if user is system admin

* Modify naming according to review comments

* Abstract getIndexDir function

* Extracting bleve engine name as a constant

* Merge both Indexer interfaces into one

* Add worker stopped message

* Allow worker to be started/stopped with config change

* Use constants for index names

* Modify test order

* Fix linter

* Trying to unlock the CI

imports/placeholder.go

@@ -9,4 +9,7 @@ import (
 
 	// This is a placeholder so this package can be imported in Team Edition when it will be otherwise empty.
 	_ "github.com/mattermost/mattermost-server/v5/plugin/scheduler"
+
+	// This is a placeholder so this package can be imported in Team Edition when it will be otherwise empty.
+	_ "github.com/mattermost/mattermost-server/v5/services/searchengine/bleveengine/indexer"
 )