Fixes fetchPosts remote service

- Fetch Post API should send the path via GET instead of POST. This is the reason why the backend call is failing, and we're not getting the associated Post's json.

- ReaderPostService is not saving fetched posts, which causes the reader to render blank posts.

Ref: Issue #1814 

/cc @astralbodies + @aerych 

(Thanks Erick for helping me out!!)

We should only really be calling saveContext here inside of the service.  The place where the service object is created should be explicitly calling the saveDerivedContext since it knows that its a derived context, not the root or main one.  The inside of the service should be pretty dumb about what it knows about the context.