Support editing assignments in Canvas

Currently we only show the editing option in non-canvas LMSes relying on canvas' built-in editing feature to allow changes in the assignment configuration.

The Canvas UI has a few limitations:

- It's not available in the context of the launch, needing to go to the assignment configuration, select and select the appropriate
- Editing this way always start from a blank slate, not remembering the previous selection of document or other settings.

Editing is not currently enabled in canvas due to it being the only LMS where we use deeplinking, meaning that the configuration (at least the source of truth) for the assignments lives in Canvas itself (in the form of the launch URL) instead of in our DB as it does in the rest of the LMSes.

We already tried a version of this in the past based on the `ext_lti_assignment_id` parameter but that didn't work out due to a number of bugs/weirdness around that value:

https://github.com/instructure/canvas-lms/issues/1952
https://github.com/instructure/canvas-lms/issues/1978
https://github.com/instructure/canvas-lms/issues/1952 (oh noes)

This needs to be revisited in the context of LTI 1.3 where, at least in some instances ext_lti_assignment_id **is** resource_link_id.