[Backend] Added validation for user role while updating or creating user

[amlannandy]

Description

Now whenever a user is created or updated, we ensure that he can only have a role of admin, management or extractor.

Fixes #114

Type of change

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

How Has This Been Tested?

Try creating or updating a user.

Test Configuration:

• Postman
• Windows 10

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules