Bulk Enrollment into Multiple Course/Memberships for Single User

Unlike #562 which adds bulk enrollment for multiple users into a single course/membership, with some tweaks to the same functionality, we can have bulk enrollment for a single user into multiple courses/memberships on `user-edit.php`, `profile.php` and `user-new.php`.

The same js function can accept `{ multiple: true }` https://github.com/actual-saurabh/lifterlms/blob/feature/bulk-enroll/assets/js/llms-bulk-enroll.js#L17

- [ ] Add a similar select2 input on the above mentioned screens
- [ ] Enqueue llms-bulk-enroll.js by adding `user`, `profile` and `user-edit` screen ids to the existing array on https://github.com/actual-saurabh/lifterlms/blob/feature/bulk-enroll/includes/admin/class.llms.admin.assets.php#L46 and https://github.com/actual-saurabh/lifterlms/blob/feature/bulk-enroll/includes/admin/class.llms.admin.assets.php#L254
- [ ] Instantiate select2 with the multiple option similar to https://github.com/actual-saurabh/lifterlms/blob/feature/bulk-enroll/assets/js/llms-bulk-enroll.js#L94
- [ ] Refactor $product_id property to handle an array of products.
- [ ] Replicate https://github.com/actual-saurabh/lifterlms/blob/feature/bulk-enroll/includes/admin/class.llms.admin.assets.php#L254 and related to create more methods to enroll single user into multiple products.

Maybe once #589 is merged, released and proven to be stable, we can start working on this one too.