Implementation details:
* Anonymous users are assigned a unique ID (like
`anon42c08f9996194e2a9339`) which gets stored in the django session.
`block.scope_ids.user_id` and `block.runtime.anonymous_student_id`
will both return this value.
* User state for anonymous users is stored in the django cache and
automatically expires as the cache gets pruned. Because user state is
stored, anonymous users can use interactive blocks like capa problems.
* There is no mechanism for upgrading to a registered account and
keeping user state since the user state store for anonymous users
(EphemeralKeyValueStore) is completely different than the one for
registered users (DjangoKeyValueStore/"CSM"), and has no "list all
keys" functionality.
* "User State Summary" field values are shared among [recently active]
anonymous users but are not shared with registered users.
* Anonymous users can only access the `public_view` of XBlocks, not the
regular `student_view`.
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