Description

While looking at https://github.com/gocodebox/private-issues/issues/63
I realized that we "try" to query the DB looking for enrollments even for not logged in users, and generally for non existing users (e.g. deleted users). In this PR I just return immediately false when checking for enrollments of non existing users when using llms_is_user_enrolled().
This is technically a breaking change (so will ideally push the next LifterLMS version to 8.0), as we're changing the default behavior of a public function although we internally always used that assuming that the user was existing: with that function you could theoretically query the enrollment status of a student in a course even if the student was deleted.
Also consider that in the future we want to delete the enrollment related data of a user upon its removal (see #940 bullet Delete data from lifterlms_user_postmeta table). Maybe we want take the occasion and implement a fix for #940 so to release a new major version including this PR?

This PR will also basically fix https://github.com/gocodebox/private-issues/issues/63

Also this PR has some perfomance impact (improvements) on not logged in users visiting e.g. courses or membership catalogs: for each course/membership in the current catalog page we check whether or not the current user (user with ID 0 for non logged in users) is enrolled in the course/membership performing a custom and not simple DB query (although on an index of the table).

How has this been tested?

manually and existing unit tests

Types of changes

Breaking change (fix or feature that would cause existing functionality to not work as expected) -->

Checklist:

• My code has been tested.
• My code passes all existing automated tests.
• My code follows the LifterLMS Coding & Documentation Standards.