Validate application while the applicant is typing

(split out from #2611)

As an applicant, it would be nice to see validation errors for the application (e.g. "invalid email", "input too long") even before pressing "save draft".

Consider running our form validations in the frontend and showing the errors to the applicant. Could hook to focus leaving an input field, or perhaps use a debounce.

- [ ] Create a prototype that runs the existing form validation in the frontend when focus leaves a field. Use the existing UI components for validation errors etc.

See https://github.com/CSCfi/rems/blob/master/docs/architecture/017-applicant-ux.md

Read docs/architecture/017-applicant-ux.md first, then locate the existing form validations and UI components used for validation errors. Prototype running the existing validation when focus leaves a field, and verify that the applicant sees errors before saving a draft.

Written by the indexing model from the issue text.