This adds new colums to represent the encrypted serialization
of `Integration#properties`.

This uses `attr_encrypted`, which adds two colums to the DB,
`encrypted_properties` and `encrypted_properties_iv`.
We use this widely for encryption, and it supports better practices such
as custom initialization vectors per attribute.

A migration is added to backfill integrations.encrypted_properties from
properties. This encrypt integrations in batches.

The new Integration model attribute is not used, and currently just
mirrors the data in `properties`, with the primary difference that it is
encrypted at rest.

We change the `integration_hash` method to re-encrypt properties on
bulk-insert, ensuring that the data remains resistant to rainbow table
attacks.

Changelog: security