Input: Additional content should get focus before the input

[silvalaura]

We recently implemented an additionalContent prop for Select to match Inputs.

In Inputs, we support this by throwing the content inside children in the upper right corner. The issue with this is that the tab order gets out of order, going from input to help link (as opposed to help link to input). This isn't helpful for screenreader users who did not know there was helpful content after they've filled in the input.

We should add an additionalContent prop to Input to match Select and NativeSelect.

We can also update our example to include aria-labeledby for the help button.