mod reports: autoreport spam and autoban spammers.

* Automatically generate a mod report when a comment, forum post, or
  dmail is detected as spam.
* Automatically ban users that receive too many automatic spam reports
  within a short window of time.
* Automatically mark spam dmails as deleted.
* Change ban threshold from 10 spam reports in 24 hours to 10 reports in 1 hour.
* Change ban length from 3 days to forever.

app/models/moderation_report.rb

@@ -7,6 +7,7 @@ class ModerationReport < ApplicationRecord
   validates :creator, uniqueness: { scope: [:model_type, :model_id], message: "have already reported this message." }
 
   after_create :create_forum_post!
+  after_create :autoban_reported_user
 
   scope :user, -> { where(model_type: "User") }
   scope :dmail, -> { where(model_type: "Dmail") }
@@ -54,6 +55,23 @@ class ModerationReport < ApplicationRecord
     updater.update(forum_post_message)
   end
 
+  def autoban_reported_user
+    if SpamDetector.is_spammer?(reported_user)
+      SpamDetector.ban_spammer!(reported_user)
+    end
+  end
+
+  def reported_user
+    case model
+    when Comment, ForumPost
+      model.creator
+    when Dmail
+      model.from
+    else
+      raise NotImplementedError
+    end
+  end
+
   def self.visible(user = CurrentUser.user)
     user.is_moderator? ? all : none
   end