FEATURE: erode bounce score every time an email is sent

Introduces a hidden setting (default is 0.1) that erodes bounce score
every time we send an email. This means that erratic failures are less
painful cause system auto corrects

db/migrate/20180828065005_change_bounce_score_to_float.rb

@@ -0,0 +1,5 @@
+class ChangeBounceScoreToFloat < ActiveRecord::Migration[5.2]
+  def change
+    change_column :user_stats, :bounce_score, :float
+  end
+end