*Background*: This PR fixes a bug related to bookmarks. If a unit is added to bookmarks and then deleted, it remains on the bookmarks page anyway. Clicking the bookmark causes a 404 error.
*openedx/core/ updates:* Bookmarks api has been extended by the `delete_bookmarks` method, which picks all bookmarks with specific `usage_key` (i.e. bookmarks made by different users but targeting the same unit), and deletes them.
*Studio updates:* The `delete_bookmarks` api method is invoked on xblock deletion (by `cms.djangoapps.contentstore.views.item._delete_item`).
The above changes have been covered by tests (3 tests added):
In cms, create a course with several units. Set up the course start date (in the past).
In lms, enroll in course, add one of the units to bookmarks.
In cms, delete the same unit that you bookmarked.
In lms, the bookmark of this unit still exists on the bookmarks page .
The same flow, but the bookmark gets deleted together with a unit it points to.