Add client-side validation in password change forms (#14564)

Add client-side validation in password change forms (#14564)

* Fix client-side username validation at registration

It used the Account::USERNAME_RE regexp which is for *remote* users,
local user validation is stricter. Also take into account max username length.

* Add client-side form validation for password change

* Add client-side form validation to dedicated registration form

Previous changes only applied to the /about page, not the dedicated form on
/auth