Popup Maker - MailChimp Integration 1.3.5

Fixes 500 error when API key is missing the hyphen
Fixes the valid_key setting requiring saving to be set to true
Fixes bug preventing the refresh button from showing
Fixes error causing some sites from being able to validate API keys

* Improvement: Migrate away from PUT and toward PATCH -> POST methodology per MC docs to be more reliable when updating existing subscribers.
* Fix: Issue where browser password filling functionality overwrites API keys.

Fix: Added test to prevent missing function error when core plugin not active during extension activation