Fixes around post data edit

Fixed filling of preview and img_url in hive_posts_data

Fixed edit case with empty title, body or json_metadata (no longer clears value, old value remains instead like it used to in fat node)

Fixed use of json packet - now everywhere ujson is used.

In case json_metadata in comment_operation would not actually be json, HiveMind no longer crashes on access. Since we are handling bad json anyway it was no longer profitable to fill empty json_metadata with {}, original empty string restored.