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 is cross-dependent on https://github.com/GeoNode/geonode-announcements/pull/2.

**This PR does not compile the new base.less code via make.  Could someone more familiar with static development commit the right compiled code.