Fixed User registration request data validation

[isabelcosta]

Description

• Added validation to the username, name, and password for max and min character limit.
• Moved request data validation functions to separate file, to avoid cluttering the resources' files with validation checks
• Added tests to check max and min limits of User registration request data fields

Fixes #94
Fixes #93

Type of Change:

• Code

Code/Quality Assurance Only

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

How Has This Been Tested?

• Added tests for character limits and used randomly generated test data.
• Tested User registration API on Swagger UI and confirmed that the error messages were showing up.

Checklist:

• My PR follows the style guidelines of this project
• I have performed a self-review of my own code or materials
• Update Swagger documentation and the exported file at /docs folder

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

[isabelcosta]

I'm looking into the Codacy warning to see if I can dismiss this

[isabelcosta]

Fixed Codacy warning of using random that brought up security concerns by using instead SystemRandom()
Solution found here: https://stackoverflow.com/a/20937265/6094482