Parsely integration not compatible with wp-parsely 3.17

[dlh01]

Description of the bug

The "Show Trending Content from Parsely" option in the query block does not work correctly with version 3.17 of the Parsely plugin. It looks like the Analytics_Posts_API class, which the integration depends on, was removed in that version (see Parsely/wp-parsely#2889).

Steps To Reproduce

Configure a WP Curate query block with "Show Trending Content from Parsely" enabled and no other parameters, and observe that the block displays a reverse-chron list of posts.

Additional Information

No response

[kevinfodness]

Looks like the class was replaced with Parsely\Services\Content_API\Content_API_Service. It seems like we might need to do the following instead of creating a new instance of Analytics_Posts_API:

$parsely_content_api = $GLOBALS['parsely']->get_content_api();
$parsely_content_api->get_posts( /* args */ );

[kevinfodness]

Also, we should ensure we support both v3.16 and below as well as v3.17 and above, so we should try to use the get_content_api method first, and if it doesn't exist, then fall back to the Analytics_Posts_API method that we were using previously, or switch to making the REST API call to Parse.ly directly, using the authentication information that the plugin provides.

[dlh01]

WIP branch: https://github.com/alleyinteractive/wp-curate/compare/fix/gh-281/parsely?expand=1