Proper keyboard support in dropdowns

[RFSH]

In react-boostrap, you can use arrow keys (or tab/shift + tab) to switch between elements in a dropdown and then use enter to select an option. You can see how the "Export" button is properly doing this. But this is not behaving properly in some of our dropdowns (like the page size dropdown).

We should search for all the dropdown usage and ensure the keyboard support works appropriately. If the reason is misusing the Dropdown component, we should document it in our dev guides so this doesn't happen again.