FEATURE: simpler and friendlier unsubscribe workflow

- All unsubscribes go to the exact same page
- You may unsubscribe from watching a category on that page
- You no longer need to be logged in to unsubscribe from a topic
- Simplified footer on emails

app/models/unsubscribe_key.rb

@@ -0,0 +1,41 @@
+class UnsubscribeKey < ActiveRecord::Base
+  belongs_to :user
+  belongs_to :post
+  belongs_to :topic
+
+  before_create :generate_random_key
+
+  def self.create_key_for(user, type)
+    if Post === type
+      create(user_id: user.id, unsubscribe_key_type: "topic", topic_id: type.topic_id, post_id: type.id).key
+    else
+      create(user_id: user.id, unsubscribe_key_type: type).key
+    end
+  end
+
+  def self.user_for_key(key)
+    where(key: key).first.try(:user)
+  end
+
+  private
+
+    def generate_random_key
+      self.key = SecureRandom.hex(32)
+    end
+end
+
+# == Schema Information
+#
+# Table name: unsubscribe_keys
+#
+#  key                  :string(64)       not null, primary key
+#  user_id              :integer          not null
+#  created_at           :datetime
+#  updated_at           :datetime
+#  unsubscribe_key_type :string
+#  topic_id             :integer
+#
+# Indexes
+#
+#  index_unsubscribe_keys_on_created_at  (created_at)
+#