RegistrationForm validation

[richardolsson]

Recent PR #275 introduces validation for the phone field in the registration form, but none of the other fields are validated. This could lead to a user assuming that a field is correct even though it's not, because they've learned from the phone field that incorrect fields will be identified by the UI. We should add validation to all other fields, as such:

• First Name must be anything but empty string
• Last Name must be anything but empty string
• E-mail must be valid e-mail
• Password must be at least six characters long