Created Unit Tests, JavaDocs, and modified some errors

[ghost]

Description

• Created unit tests for the FirebaseSystem, Controller, Data, Interactor, Response, and Presenter Class in the Profile Customization Use Case

• Created JavaDocs for all the methods in the profile_customization_use_case and also the methods in DBService that Profile Customization uses

Fixes # (issue)

• Changed CustomizationGatewayImplementation to CustomizationFirebaseSystem to match the proper naming convention

• Modified the existName method in DBService so that it correctly checks the newly chosen name instead of userID which was what it was checking before

Design Patterns

Please mention which - if any - design patterns that you have implemented in this pull request.

SOLID Principles

Please mention which - if any - SOLID principles you have followed in this pull request.

Breaking Change

• [ ] Check this if your changes will cause merge conflicts in other branches.

Checklist:

• [x] My code follows Java style conventions
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have added Javadocs for new classes and methods
• [x] My changes generate no new relevant warnings
• [x] I have added tests for any new functionality added
• [x] New and existing unit tests pass locally with my changes