Checkout notice for already enrolled users should link to the product instead of the dashboard

**Is your feature request related to a problem? Please describe.**

> It’s just a bit tedious for users to have to go to their Dashboard instead of going straight to their course page (I’m talking about the link in “Visit your dashboard [here]”).

From https://wordpress.org/support/topic/add-filters-for-you-already-have-access-text/

**Describe the solution you'd like**

Change the link in the message to link to the product (course or membership) permalink.

We should add a filter here as well (as requested by user in .org thread).

**Describe alternatives you've considered**

User has requested a filter on the text.

Here's the relevant lines of code:

https://github.com/gocodebox/lifterlms/blob/bd5810133f9ee53c687384aba1a8006d19ba5f33/includes/shortcodes/class.llms.shortcode.checkout.php#L69-L80

Related to (now closed) bug #607