Updated cancel page to be a nonced form with new actions

[dparker1005]

All Submissions:

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

Changes proposed in this Pull Request:

The goal of these changes is to allow Add Ons (Such as the Reason For Cancelling Add On) and custom code to easily hook into the cancel page.

Added new actions:

• pmpro_cancel_before_submit
• pmpro_cancel_should_process
• pmpro_cancel_processed

Other information:

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

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

[dparker1005]

The diff looks messy due to fixed indentation. This may break backwards compatibility for custom page templates, but this is why it is important to release in a major version. It also adds new filters which should help avoid the need for custom templates going forwards.