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.

cookbooks/unbound/renovate.json

@@ -0,0 +1,6 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "config:base"
+  ]
+}