refactor: centralize checks for canonical courseware experience & URL (#26815)

Centralize the logic for choosing between
MFE and Legacy-frontend courseware within
three new functions:
* courseware_mfe_is_active
* courseware_mfe_is_visible
* courseware_legacy_is_visible

This allows us to create another new function:
* get_courseware_url
which can be called anywhere in LMS/Studio
to get the canonical URL to courseware
content (whether it be MFE or Legacy).

In future commits we we begin using
get_courseware_url throughout the platform.

TNL-7796