[WIP] HTML5 validation for formintinput

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | -
| License       | GPL
| Doc PR        | -

I know that the formplugins are legacy, but it would be nice to have it for the moment.