FEATURE: allow users to control how many previous replies they get

- always means we always send previous replies with every email
- never means we do not
- "unless previously sent" ... is the default, in which we only email you each reply once

The default_email_previous_replies site setting can control this toggle

app/models/previous_replies_site_setting.rb

@@ -0,0 +1,22 @@
+require_dependency 'enum_site_setting'
+
+class PreviousRepliesSiteSetting < EnumSiteSetting
+
+  def self.valid_value?(val)
+    val.to_i.to_s == val.to_s &&
+    values.any? { |v| v[:value] == val.to_i }
+  end
+
+  def self.values
+    @values ||= [
+      { name: 'user.email_previous_replies.always',  value:  0 },
+      { name: 'user.email_previous_replies.unless_emailed',   value:  1 },
+      { name: 'user.email_previous_replies.never', value:  2 },
+    ]
+  end
+
+  def self.translate_names?
+    true
+  end
+
+end