LibraryElements: Creates usage stats for panels and variables (#34476) (#34555)

* LibraryPanels: Adds usage collection

* Refactor: renames Panel and Variable consts

* Chore: initialize stats

* Refactor: moves library element migrations to migration namespace

(cherry picked from commit 7204a64717)

Co-authored-by: Hugo Häggmark <hugo.haggmark@grafana.com>

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) {