Show "Mailchimp" customer tab when the extension is enabled

[t-richards]

This PR fixes the page layout for Customer > Index > Edit so that the "Mailchimp" tab is only displayed when the extension is enabled.

As a side effect, attempting to load the customer's interests no longer happens, which should fix #200 .

[gonzaloebiz]

Hi @t-richards

first of all, thanks for your feedback.
The main idea is OK, the tab must be displayed only when the extension is enabled and your PR fix it.
But i must say that i can't replicate the issue #200
So, you can please change the subject of the PR, that way we maintain documented your PR

Best

[t-richards]

The PR title has been changed.