vicfg: Deleting configurations now resets to the default, where before it has no effect.

Added a Default instance for TrustLevel, and was able to use that to clear
up several other parts of the code too.

This commit was sponsored by Stephan Schulz

Types/StandardGroups.hs

@@ -9,6 +9,7 @@ module Types.StandardGroups where
 
 import Types.Remote (RemoteConfig)
 import Types.Group
+import Data.Default
 
 import qualified Data.Map as M
 import Data.Maybe