[BTR-53] [BB-3502] Misuse of urljoin in Gradebook URL from Instructor Panel

[spokerman12]

When the Gradebook MFE is deployed on a subdirectory, urljoin returns the wrong link on Instructor Dashboard.

JIRA tickets: OSPR-5358 BTR-53

Discussions: Background: https://github.com/edx/configuration/pull/6171#discussion_r549190558

The frontend-app-gradebook doesn't use the /gradebook/ path out of the box (or any other). The present issue is specified as "the Gradebook link [stops] working if Gradebooks are deployed using the subdirectory mechanism," based on the linked discussion. From this method's code and its unit test, the frontend-app-gradebook should be hosted at WRITABLE_GRADEBOOK_URL. I believe these test cases (based on the mfe_deployer role in configuration/master) should pass

    WRITABLE_GRADEBOOK_URL = https://MFE_BASE/gradebook
    WRITABLE_GRADEBOOK_URL = https://MFE_BASE
    WRITABLE_GRADEBOOK_URL = https://any.domain.really

Using .format instead of urljoin works nicely, and it's also applied on some important edx-platform code with similar behavior.

Dependencies: None

Screenshots: See "Testing instructions"

Sandbox URL: Sandbox. You can use staff@example.com to access the Demo Course's Instructor panel. This instance was deployed with configuration's master branch

Merge deadline: None

Testing instructions:

• Deploy a devstack using master
• Set up a course and an admin account if there isn't one.
• Enable Gradebook MFE using the instructions here.
• This will have you set WRITABLE_GRADEBOOK_URL: 'http://localhost:1994' on /edx/etc/lms.yml, but because we want the MFE on a subdirectory, let's set it to http://localhost:1800/gradebooks (this is based on how the MFE deploy role on configuration does it).
• Go to your course's instructor panel as staff@example.com or similar.
• Check the link. Note the URL, it's missing /gradebooks/

After applying the fix:

Author notes and concerns: None.

Reviewers

• @farhaanbukhsh
• felipemontoya, morenol