MM-9728: Online migration for advanced permissions phase 2 (#8744)

* MM-9728: Online migration for advanced permissions phase 2

* Add unit tests for new store functions.

* Move migration specific code to own file.

* Add migration state function test.

* Style fixes.

* Add i18n strings.

* Fix mocks.

* Add TestMain to migrations package tests.

* Fix typo.

* Fix review comments.

* Fix up the "Check if migration is done" check to actually work.

imports/placeholder.go

@@ -4,3 +4,7 @@
 package imports
 
 // This is a placeholder so this package can be imported in Team Edition when it will be otherwise empty
+
+import (
+	_ "github.com/mattermost/mattermost-server/migrations"
+)