Submit sketches to someone else's collection

Nature of issue?

• New feature request

New feature details:

When collections have been released (#1115) users will be able to add any user's sketches to their own collections. For classroom/assignment use cases, it would be great for other users to be able to submit sketches to be included in a collection.

Example workflow:

I'm using teacher and student in this example, but it can be any two editor users. The important thing is that one of them will own the collection and the others will submit sketches to it.

1. A teacher creates a new collection and shares the URL with their students
2. A student uses that URL to load the collection
3. The student then selects a sketch and submits it to the collection
4. The teacher receives a notification that a new sketch has been submitted
5. If they "accept" the submission, it is added to the collection. If they "deny" the submission, it will not be added to the collection.
6. The student is notified of the status of their submission

Start by reviewing the collection work referenced in issue #1115 and trace how collections are currently created, shared, and updated. Define the submission, acceptance or denial, and notification flow, then verify that students and collection owners receive the appropriate status and that accepted sketches appear in the collection.