Validation for S3 UI fields

Closes #30 

Uses the new i18n system for error messages, and HTML5 form validation attributes.
Yeah forget the library, it was a bad suggestion. If there is a way of displaying an error message using paper-input's built in reporting that also allows arbitrary validation functions instead of single regexes, I'm all for it. Otherwise, the best solution is to test the contents of each field with a function manually and display a toast.