Apply Application Factory pattern

[isabelcosta]

Description

• Applied Application Factory pattern, which does lazy initialization for the flask extensions, and allows to create the application with a single function create_app()
• This function is used while running the app, and when setting up API testing
• Fixed imports

Fixes #55

Type of Change:

• Code

Code/Quality Assurance Only

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• I ran the application with python run.py
• I created a test for POST /register which worked as expected

Checklist:

• My PR follows the style guidelines of this project
• I have performed a self-review of my own code or materials
• I have commented my code or provided relevant documentation, particularly in hard-to-understand areas
• Any dependent changes have been merged

Code/Quality Assurance Only

• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes