Resolve problems pushing to and pulling from Story API

[benlk]

Changes

This is a wrap-up of miscellaneous errors noticed when diagnosing problems pushing to the API, documented in #57.

• Fixes issue where action used to push posts to the API was not running, because of a missed spot in https://github.com/npr/nprapi-wordpress/pull/47 identified in #57
• Fixes issue where images were not properly sideloaded when pulling from the API, identified in #59
• Removes duplicate <hr/> seen in the query-based fetch settings page "NPR API Get Multi", which was between the "Publish Stories X" and "Add Tags X" settings.
• Fixes an "undefined offset" PHP notice that occurred when attempting to delete a post from the API when the post had no API ID.
• Fixes an "undefined variable" PHP notice that occurs when loading the "Get NPR Stories" page without a post specified
• Fixes a "Non-static method DS_NPR_API::load_page_hook() should not be called statically" PHP notice that appears on the "Get NPR Stories" page — because the method was hooked incorrectly. And changes a test to match.
• Fixes an "Uncaught Error: Cannot use string offset as an array" error in the plugin settings page when checking API permissions. https://github.com/npr/nprapi-wordpress/issues/57#issuecomment-424903460

Why

Prevents a couple issues when pushing or pulling posts from the API.