Clear cache when Mailchimp API key gets updated

Problem/Motivation

mailchimp_get_lists() caches Mailchimp lists in order to not build the list structure on each request.
However, when a user changes Mailchimp API key on the configuration form can still contain mailchimp_get_lists() stale items.

Proposed resolution

Flush Mailchimp cache when a user changes Mailchimp API key on the configuration form.

Remaining tasks

User interface changes

API changes

Data model changes