Add additional checks for course enrollment limit reached

[brianhogg]

Is your feature request related to a problem? Please describe.
Currently, there is only a check if the course enrollment limit has been reached when showing the pricing table. If two people are on the course access plan checkout page already and there is only one spot left, they are both able to check out.

Describe the solution you'd like
Add an additional check on the checkout page and before processing payment (or completing a free plan order) to see if the course enrollment limit has been reached. Also check before redirecting to an off-site payment method (ie. paypal).

This is not 100% since we don't want to take payment (ie. PayPal) and then refund it, costing money in processing fees without permission from a site admin.

[brianhogg]

Verify how the check is being done, if using a value that isn't re-calculated immediately. We may want to add a job to query to re-calculate immediately.

[brianhogg]

Also need to consider WooCommerce checkout.