Make the Wizard previous and next buttons focusable again

[afercia]

Summary

This PR can be summarized in the following changelog entry:

• Made the Onboarding Wizard previous and next buttons focusable again

Relevant technical choices:

• Removes the disableKeyboardFocus from the buttons. Seems this prop was initially used in material-ui just for styling purposes, instead now it's also responsible for setting the tabindex attribute.

Test instructions

• on the develop branch, use the keyboard to Tab to the buttons and see they're not focusable
• see they have a tabindex="-1" attribute
• switch to this branch
• check the buttons are now focusable
• check they have a tabindex="0" attribute

Aside note: as mentioned in the issue, material-ui does lots of things to control elements focusability programmatically. Not sure I really like this approach. Keeping controls behaviors as native as possible is always a better choice.

Fixes #368

[abotteram]

CR 👍

[abotteram]

Acceptance 👍