Users module HTML5 form validation

Switching to HTML5 for validation of the user registration form. Refs. https://github.com/zikula/core/issues/1181

Webshim will need to be added for a polyfill solution.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | -
| License       | MIT
| Doc PR        | -