Sessions generator: add basic registrations

[yshmarov]

#52328 adds sessions (existing user can sign in).

This PR adds registrations (new user can sign up).

We already have database uniqueness validation for email_address, now we add validations in the User model to display errors during registration.

---

I think the next step to make this new authentication solution viable for a real app would be adding "forgot/reset password" feature.

[skipkayhil]

The filename was changed in #52435, can you please rebase?

[dhh]

Appreciate the work here, but I don't like the idea of a top-level registrations controller. Maybe it's just the naming, but most apps have a great degree of variety there. There are SignupControllers or AccountControllers or UserControllers. Will think about this a bit more on how it might fit in.

Also don't like using validations for error handling. HTML has finally given us email validation straight on the input tag. Don't want to replicate that.

[yshmarov]

Appreciate the work here, but I don't like the idea of a top-level registrations controller. Maybe it's just the naming, but most apps have a great degree of variety there. There are SignupControllers or AccountControllers or UserControllers. Will think about this a bit more on how it might fit in.

Also don't like using validations for error handling. HTML has finally given us email validation straight on the input tag. Don't want to replicate that.

I do use email_field to leverage the HTML email validation.

I view the model validation as an additional safety factor against malicious users that try to manipulate the data submitted in the form :)