Add Postgres and MySQL databases support (#335)

internal/db/user.go

@@ -6,7 +6,7 @@ import (
 
 type User struct {
 	ID        uint   `gorm:"primaryKey"`
-	Username  string `gorm:"uniqueIndex"`
+	Username  string `gorm:"uniqueIndex,size:191"`
 	Password  string
 	IsAdmin   bool
 	CreatedAt int64