Cleanup of module POST.

* [GENERICS,EXT] Extended the HTTP response generator with three status codes.
* [NEWS,SC] Simplification of code due to POST changes.
* [POST,NEW] API for post properties.
* [POST,SC] Use of JSON conversion for JSON<-->PL translation.
* [POST,EXT] Added response codes for RESTful predicates.
* [POST,SC] Shared function for adding and editing posts.