LibraryElements: Creates usage stats for panels and variables (#34476)

* LibraryPanels: Adds usage collection

* Refactor: renames Panel and Variable consts

* Chore: initialize stats

* Refactor: moves library element migrations to migration namespace

pkg/services/sqlstore/migrations/migrations.go

@@ -40,6 +40,7 @@ func AddMigrations(mg *Migrator) {
 	addShortURLMigrations(mg)
 	ualert.AddTablesMigrations(mg)
 	ualert.AddDashAlertMigration(mg)
+	addLibraryElementsMigrations(mg)
 }
 
 func addMigrationLogMigrations(mg *Migrator) {