Add model validations and corresponding rspec tests

- Add validations for uniqueness and presence of both username/email in User model
- Add validations for uniqueness and presence of both name/email in Artist model
- Add rspec test for uniquess and presence of username/name and email in both User and Artist model specs
- Add validation for email format in both User and Artist model
- Add rspec test for legit email format in both User and Artist model specs
- Currently, 21 tests passing