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


------------------
AC + CT
Ah, thank you!  I didn't realize that it did that automatically.  The single argument should be it's "purpose" and is simply an additional salt on top of the rails secret.  So, we'll change this to be some ihat verifier symbol.