Set up SpamAssassin

Scan incoming and outgoing email for spam. Use a local Unbound for DNS,
so we don't run into blocks for RBL queries.

Berksfile

@@ -37,6 +37,7 @@ cookbook 'timezone_iii',           '= 1.0.4'
 cookbook 'ulimit',                 '~> 1.0.0'
 cookbook 'users',                  '~> 5.3.1'
 cookbook 'zerotier',               '~> 1.0.7'
+cookbook 'unbound',                '~> 3.0.2'
 
 # openresty dependency
 cookbook 'jemalloc',               '~> 0.1.7'