In addition to migrating password fields, this also removes the remaining
support for old-style validation in registration now that all checks have been
converted.
When submitting a form, we want to validate more strictly to check for empty
values that might be required. A separate mode is used since we want to ignore
this issue when visiting a field one by one to enter data.
As an example, we convert the pre-existing logic for the username requirement
using this new support.
This updates the registration form to include the new-style validation state
when deciding whether the entire form is valid overall.
In addition, this tweaks the validation helper to take functions instead of
strings for translated text. This allows the validation helper to be create once
per component instead of once every render, which improves performance.
