Hide confirmation message from checkout

[andrewlimaza]

• ENHANCEMENT: Ability to show the confirmation message if the user isn't approved. If the user is already approved, they will see the unedited confirmation message.

Use the filter pmpro_approvals_show_confirmation_message and return true if you want non-approved members to see this context.

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?

[andrewlimaza]

Resolves: #173