mailchimp 7.x-2.12

This is a quick follow up release addressing a couple issues that arose in 2.11. We also snuck in a few new features for everyone's trouble ;)

Changes since 7.x-2.11:

• #1960616 by levelos: Add a hook for acting on webhook processing.
• #2049273 by levelos: Remove the created and updated timestamp fields from mailchimp_lists.
• #1950408 by levelos: Add a check for the library being installed before returning the API object in mailchimp_get_api_object().
• #2049237: MailchimpList::label() calls itself.
• Add a test for deleting lists.
• Use our new mailchimp_list_create() helper in tests.
• Add a function to create new MailchimpLists.
• #2048755 by levelos: When saving a new list, instantiate it as a MailchimpList class rather than stdClass.