Add DMCA complaint form.

Add a form for submitting DMCA complaints. The complaint is emailed to
the site owner, and a confirmation email is sent to the submitter.

config/danbooru_default_config.rb

@@ -91,6 +91,11 @@ module Danbooru
       "webmaster@#{Danbooru.config.hostname}"
     end
 
+    # The email address where DMCA complaints should be sent.
+    def dmca_email
+      "dmca@#{Danbooru.config.hostname}"
+    end
+
     # The email address to use for Dmail notifications.
     def notification_email
       "notifications@#{Danbooru.config.hostname}"

config/routes.rb

@@ -296,6 +296,9 @@ Rails.application.routes.draw do
       get :diff
     end
   end
+  resource :dmca, only: [:create, :show] do
+    get :template
+  end
 
   # Legacy Danbooru 1 API endpoints
   get "/tag/index.xml", :controller => "legacy", :action => "tags", :format => "xml"