Purge Now button not purging WPE cache

[Mark Kelnar]

In the WPGraphQL admin settings page, when the 'Purge Now' check box is selection under 'Cache' and the settings are saved, the 'wpgraphql_cache_purge_all' action is triggered. This action should trigger the WP Engine varnish network cache to invalidate all WP GraphQL data for the domain.

[Jason Bahl]

closed by #194