Provides fields validation for the User model.

#### What's this PR do?
- Adds [Laravel Model Validation](/JeffreyWay/Laravel-Model-Validation) composer package
- Introduces user fields validation
- Makes `Email` unique as required per #10. Username is generated from email, so it'll be unique as well

#### Any background context you want to provide?
Composer package added, so `composer update` is required during deployment

#### What are the relevant tickets?
Laravel part for issue #10.