Add encryption support to `use_user_data` hook

[Archmonger]

Current Situation

Our use_user_data hook stores user data as unencrypted binary. If the user wants to encrypt certain key/values, this needs to be done manually which is highly inconvenient.

Proposed Actions

Add built-in encryption support to this hook. This design will be inspired by django-searchable-encrypted-fields.

Based on that library, here's a few specifics about the implementation:

1. Create settings.py:REACTPY_USER_DATA_ENCRYPTION_KEYS: list
2. Add encrypted = models.BooleanField(default=False) to UserDataModel and ComponentSession
3. Add new manage.py reactpy_upgrade_data_encryption management command.
   • Encryption keys are upgraded lazily by default, but this command will be used to manually upgrade all data to the newest encryption key.
4. Add a check to manage.py check --deploy that warns if encryption is not enabled.
5. Warn if encryption keys are too short or lack entropy.