Allow Notebooks to be associated with many users (but one owner).

This is done through a many-to-many relationship in the database, and
a single `owner` field/column in the Notebook model/table. If we
decide later to have multiple admins for a single Notebook this will
need to change, but this works for now.
Make Notebooks work again with the new schema.

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Allow users to share notebooks.

Right now some (basic) permissions are enforced, but a better approach
is needed to make sure that only the owner of a notebook can edit its
structure.
Update tests to pass with notebook sharing.