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!!)

Surely!  Calling saveContext is perfectly fine in a service since it's not assuming anything about the context.  Just make sure to add the saveDerived outside :smile: