[EuiHeaderAlert] Updated to new styles for Kibana news feed

[ryankeairns]

The 'Whats New?' news feed is currently being added to Kibana and it relies on an old EuiHeaderAlert component that, to this point, has never been used or documented. The current plan is to add the component directly to Kibana, temporarily, until we get this component cleaned up and in working order on the EUI side.

What needs to happen?

• Make sure that EuiHeaderAlert is being properly exported/can be imported to a page
• Add docs for EuiHeaderAlert to the current EuiHeader docs page (i.e. add a new section; add header button with mail icon that opens flyout; display EuiHeaderAlert's in the flyout) -- see marvel for reference https://marvelapp.com/1d65j85g/screen/53663628
• Apply the revised markup and styles from the prototype https://codesandbox.io/s/l9pwrw289
• Add a prop that accepts an optional badge. The badge can be customized for color and text, so perhaps it should accept an EuiBadge like badge={<EuiBadge>7.1</EuiBadge>}? or else it may just accept two separate props for badgeText or badgeColor?