Ihat callbacks

### References: 
* https://www.pivotaltracker.com/story/show/83452714

### Heads up
* remove `IHatJob` model
* no longer make a subsequent request back to iHat to get converted file (just request it directly)

### Review, but DO NOT MERGE until iHat changes are pushed out


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AC + CT
Since you're using `Rails.application.message_verifier`, you don't need to provide `ENV.fetch("RAILS_SECRET_TOKEN")`, it will use that token internally. I don't know what other thing makes sense though, since I know that `#message_verifier` takes one argument.