When site has no GMT offset saved, the expiry datetime causes problems

[benlk]

When the 'gmt_offset' value is a number (which is pretty much everywhen), the DateTimeZone constructor complains that the number is not a valid timezone.

https://github.com/npr/nprapi-wordpress/blob/853a3011aafa9727a8742111305ff3194abca236/push_story.php#L676-L679

PHP Fatal Error: Uncaught exception 'Exception' with message 'DateTimeZone::__construct(): Unknown or bad timezone (-4)' in /plugins/npr-story-api/psuh_story.php:676'

• [ ] make plugin draw from 'timezone_string' when 'gmt_offset' is unset, in function nprstory_get_post_expiry_datetime() and in function nprstory_save_datetime()
• [ ] provide fallback behavior for when those meta are not set
• [ ] add test case that runs nprstory_get_post_expiry_datetime( $post ) with every valid GMT offset and with every time zone and with nothing set for either.
  • [ ] when running against every time zone, the list at https://secure.php.net/manual/en/datetimezone.listabbreviations.php will need to be wp_list_pluck'd to get the timezone_id value, and then will need to be sort/uniq'd.

[benlk]

Timezones are not a specific GMT offset; we need a GMT offset for this. A better approach is described in https://github.com/npr/nprapi-wordpress/pull/53 that makes use of WordPress's tools for converting timezones to GMT offsets.

[kaylima]

TODO: Up to 2 hours for remaining testing for #53 .

[benlk]

Automated tests for #53 pass.

Human-run tests for #53 should perform the following tasks:

1. Activate the plugin
2. Configure the plugin with test API credentials
3. Create a new post.
   1. Does the NPR Story API meta box appear on the page?
   2. Are there no errors in the server console?
   3. Change the time zone in Dashboard > Settings > General to a new time zone. Repeat step 3 until satisfied.
4. Push a post to the API.
5. Pull a post from the API.