Having these in a hash in the SidekiqProcessor has a few problems:

1. It's very far away from the definition of `perform`, so it's hard to
   verify.
2. That also makes it harder to notice.
3. The big hash is more prone to conflicts or ordering issues.

This adds a `loggable_arguments` method to `ApplicationWorker`. When
called with any arguments, it acts as a getter. When called with no
arguments, it acts as a getter.

The arguments have to be declared by position, because we don't know
what they're called in Sidekiq. There is also no need to mark numeric
arguments as loggable as they are loggable anyway.