Announcement banners can now be dismissible

Announcement banners allow Grafana administrators to share status updates to their users, for example when there is an ongoing incident or scheduled maintenance. Particularly for more informational updates, users often wanted a way to acknowledge and dismiss the banner so it was no longer in the way. Now that is possible. Administrators can optionally toggle whether a banner is dismissible by a user. A banner stays dismissed across all sessions and browser windows, unless an administrator updates the banner - in which case it reappears.