`videosequence` and `problemset` have been replaced with `sequential`.
`problemset` and `videosequence` are old-but-not-entirely-unused aliases to the `sequential` block type (in Studio-speak, "Subsection").
Since [these block types have been removed from the 6 courses that used them](https://openedx.atlassian.net/browse/DEPR-151?focusedCommentId=588197), this ticket removes the support for the `problemset` and `videosequence` block-types.
For more information, see ticket: [DEPR-151](https://openedx.atlassian.net/browse/DEPR-151)
* apply policy per-course, even if multiple courses share course and org fields
* keep descriptors separate in xml store, so that if two such courses
change the same module in different ways, it works. Such edits will need to
merged on CMS import...
* add get_instance(course_id, location) method to replace get_item(location).
Update all the call sites
* tests, including a 2nd toy course with same course and org.
