This DRYs up setting up encrypted columns for a model. You just
need to specify the column or columns to be encrypted, and the
plugin takes care of setting up the column_encryption plugin.
Other advantages:
* Model.redacted_columns is now an attr_reader, and does not
need to check for encrypted columns every time it is called.
* Model#before_destroy can look at Model.encrypted_columns (also
an attr_reader), so it is simplified as well. Simplify the
method while here by using hash key omission, and make sure
the constant it uses is frozen.
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