Change "Cancel" btn to "Cancel auto-renew" when subscription is still active

[mircobabini]

All Submissions:

• [x] Have you followed the Contributing guidelines?
• [x] Does your code follow the WordPress' coding standards?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Resolves #16.

How to test the changes in this Pull Request:

1. Make a recurring level order
2. You should now see "Cancel auto-renew" instead of "Cancel"
3. After you use it, you gonna see "Cancel" to completely cancel the membership

➕ we should provide a snippet to completely remove the cancel button ➕ we should avoid double-clicks on cancel button to cause complete membership cancellation (js hack)

Other information:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] Have you successfully run tests with your changes locally?

Changelog entry

Change "Cancel" btn to "Cancel auto-renew" when subscription is still active