Replace Donation Banner with a generic Notification Banner to use when necessary

[obulat]

Fixes #52

The CC donation banner and the a/b test for its language need to be removed from the site.

This PR converts the DonationBanner into a more general component to display a site banner beneath the header navbar whenever we need that. It creates a new store to keep the state related to the notification banner.

To create and show the Notification Banner, you need to:

• set showNotification in the notification-store state to true.
• If you need an 'Okay' action button, set header.notification.okay in the locale file to the name of the button, and add the action that needs to be performed in the NOTIFICATION_ACTION in the notification-store.
• If you want the Dismiss button to have a text and not just a cross icon, add the button text to header.notification.dismiss key in the locale file.

The test failures in the a/b experiment tests prevented me from pushing, so I fixed them, too.