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)

### When to deploy?
This can be deployed once the following iHat pull request is deployed:
* https://github.com/Tahi-project/ihat/pull/29


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AC + CT