enable rspamd's autolearn feature

core/rspamd/conf/classifier-bayes.conf

@@ -0,0 +1,6 @@
+autolearn {
+  spam_threshold = 6.0; # When to learn spam (score >= threshold)
+  ham_threshold = -0.5; # When to learn ham (score <= threshold)
+  check_balance = true; # Check spam and ham balance
+  min_balance = 0.9; # Keep diff for spam/ham learns for at least this value
+}

towncrier/newsfragments/2446.bugfix

@@ -0,0 +1 @@
+Enable rspamd's autolearn feature to ensure that its bayes classifier has enough HAM to make it usable. Previously the bayes module would never work unless some HAM had been learnt manually.