Add JobIteration::Deprecation

[sambostock]

This adds a Deprecation object using ActiveSupport::Deprecation, allowing us to log fancy deprecation warnings.

It has been extracted from #81, as it will also be relevant in other PRs.

I believe the missing piece #81 needed to properly work with DeprecationToolkit was that we not only needed to configure it to attach_to our deprecations, but we also needed to add our Deprecation to the host application's deprecators, so DeprecationToolkit will add the :notify behavior to it, so it has notifications to subscribe to.¹ The simplest way to do this is for us to use a Railtie to register an initializer which does it.

Footnotes

1. I feel like there's some documentation that could be improved, either in Rails or in DeprecationToolkit to capture this. ↩

[sambostock]

It would be nice to have tests for this that spin up a full Rails application (see #459), but in the interest of unblocking some other work, we're going to merge this as is and rely on manually testing the deprecation code paths in host apps for now.