Rename maintenance.rb -> danbooru_maintenance.rb.

Fixes random test failures caused by ambiguous constant lookup issues
(the `Maintenance` module name was used in multiple conflicting places).

lib/tasks/maintenance.rake

@@ -3,16 +3,16 @@ require "tasks/newrelic" if defined?(NewRelic)
 namespace :maintenance do
   desc "Run hourly maintenance jobs"
   task hourly: :environment do
-    Maintenance.hourly
+    DanbooruMaintenance.hourly
   end
 
   desc "Run daily maintenance jobs"
   task daily: :environment do
-    Maintenance.daily
+    DanbooruMaintenance.daily
   end
 
   desc "Run weekly maintenance jobs"
   task weekly: :environment do
-    Maintenance.weekly
+    DanbooruMaintenance.weekly
   end
 end