Detect and prevent image bombs, max. processable dimension 4096^2 (#7229)

app/lib/exceptions.rb

@@ -6,6 +6,7 @@ module Mastodon
   class ValidationError < Error; end
   class HostValidationError < ValidationError; end
   class LengthValidationError < ValidationError; end
+  class DimensionsValidationError < ValidationError; end
   class RaceConditionError < Error; end
 
   class UnexpectedResponseError < Error