Write a plan for changing `sections_enabled` to an assignment setting

Create a plan and proof-of-concept for changing `sections_enabled` into a simple per-assignment setting manually chosen by the instructor when creating or editing an assignment.

* When creating an assignment the teacher should be able to choose whether it will be a course, group or sections assignment.
* Ideally the currently used code for automatically deciding whether an assignment is a sections assignment based on application instance and course-level settings should be removed, or at least made into legacy code.
* The group and sections options must only appear if those features are enabled on the application instance.
* If we disable `ApplicationInstance.sections_enabled` any existing sections assignments should remain as sections assignments and should fail to launch (because we'll no longer be asking for the sections scopes when getting access tokens from Canvas)

See also:

* [Choose whether assignment is course, section, or group](https://github.com/hypothesis/product-backlog/issues/1309)