[Improvement] Find ways to incorporate design patterns into our program

[Lei-Tin]

As requested in the Milestone 4 Rubrics, we can try to find ways and include design patterns in our code.

Current design patterns

At of the current moment, we only have the Builder design pattern for entries & Singleton design pattern for Users. We can think of other ways to incorporate design patterns into our program.

Regarding MySQLDatabaseGateway

Also, I would like to claim that the MySQLDatabaseGateway is in fact, a Strategy design pattern. We have the DatabaseGateway as the Strategy, and we are implementing it concretely by the MySQLDatabaseGateway.

If we decide to extend this to some other database interfaces such as MongoDB or MariaDB, then we can simply create another strategy to include this new strategy.