Actually make use of redux-offline and use it to make _all_ our requests

[randytarampi]

Use some combination of https://github.com/redux-offline/redux-offline#3-decorate-actions-with-offline-metadata and https://github.com/redux-offline/redux-offline/blob/develop/docs/recipes/customize-requests.md#the-effect-reconciler to rejig the way we currently fetchPosts, fetchResume and fetchLetter.

The trouble here is that we're currently dispatching thunks 'cause there some response handling work that we need to do that really isn't appropriate to put in the reducer (at least, that's my feeling), like casting the response to our desired Immutable Record type.

Exactly how we go about doing this isn't clear – but I'm thinking that we rejig the reconciler to take a reference to some function that runs after the response is received and transforms it into something worth dispatching in the commit or rollback action.

We basically want to use this, or something similar.