There are (at least) two changes that are breaking:
- defaultTab is gone
- The re-rendering logic looks to have been re-written so that
  passing in a new array of tabs causes a re-render, which can
  reset the currently selected tab.

The fixes involved removing defaultTab from each tab configuration,
as it is no longer respected anyway. Also, instead of letting Tabs
be uncontrolled and allowing it to set its own selected, which
would result in the first tab automatically being selected on
Tabs re-render, use Tabs a controlled prop to dicate which
tab is selected; this is accomplished by specifying a selected
prop.

This is a pre-requisite to updating atlaskit/tabs to 8.0.8.
Without updating, clicking a dropdown menu within a tab
component within a modal, such as the language selector in
the settings dialog, will lock the browser.

* Welcome page calendar ui improvements

* Addressing PR review comments.

Very likely I broke something subtle and I'm prepared to fix it.