Announcement Types/Levels - Initial

Initial support for setting the announcement type/level.  Users can now choose among a list of types for an announcement: general, warning, critical, and super critical.  These types are a combination of two values: severity and urgency.  A higher severity leads to a more noticeable warning.  In a future PR, an urgent warning can lead to automated actions, such as emailing users, push notifications, etc.  The CSS is easily extensible.  Announcements with a "general" type display the same as current announcements.

The foreground and background color of the announcement banner changes with the severity of the announcement.  This PR relies on pending PR to https://github.com/GeoNode/geonode-announcements.

**This PR does not compile the new base.less code via make.  Could someone more familiar with static development commit the right compiled code.