migrate: Fix 'no such table: migrations' err

Apparently, if you CREATE TABLE inside of a transaction, and then
(in the same transaction) do a SELECT on the same table before
committing, the table will not exist yet.

Now we do the migration in two steps: first initialize the schema;
then migrate the data.

cmd/rushlink-migrate-db/main.go

@@ -33,14 +33,15 @@ func main() {
 		log.Fatalln(err)
 	}
 
+	// Migrate database schema
+	m := db.Gormigrate(sqlDB)
+	if err := m.MigrateTo("202010251337"); err != nil {
+		log.Fatalln(err)
+	}
+
 	// Migrate all files in filestorage
 	if err := sqlDB.Transaction(func(sqlTx *gorm.DB) error {
 		return boltDB.Bolt.View(func(boltTx *bolt.Tx) error {
-			// Migrate database schema
-			m := db.Gormigrate(sqlTx)
-			if err := m.MigrateTo("202010251337"); err != nil {
-				return err
-			}
 			// Migrate all the file uploads
 			allFUs, err := boltdb.AllFileUploads(boltTx)
 			if err != nil {