Improve keyboard support for availability tabs

[mdarrik]

I mentioned this in the PR, but the tab controls currently need a little keyboard support work.

If you look at the w3 example I based the tab changes on, it does some logic to make the content for a tab available at the next tab index after a user clicks a tab. It also add a some arrow key support so users can switch between tabs more easily.

When you do this, you may want to break the tab stuff out into components like: tab-list, tab-button, tab-content so that this could be reused elsewhere.