Read 2L-KMS encrypted sessions

[jmhooper]

Why: To start the migration to 2L-KMS. This commit wraps an AES
encrypted ciphertext with KMS. This commit allows new and old instances
to exist in parallel when we start writing 2L-KMS encrypted ciphertexts
to the session.

Hi! Before submitting your PR for review, and/or before merging it, please
go through the following checklist:

• For DB changes, check for missing indexes, check to see if the changes
  affect other apps (such as the dashboard), make sure the DB columns in the
  various environments are properly populated, coordinate with devops, plan
  migrations in separate steps.

• For route changes, make sure GET requests don't change state or result in
  destructive behavior. GET requests should only result in information being
  read, not written.

• For encryption changes, make sure it is compatible with data that was
  encrypted with the old code.

• For secrets changes, make sure to update the S3 secrets bucket with the
  new configs in all environments.

• Do not disable Rubocop or Reek offenses unless you are absolutely sure
  they are false positives. If you're not sure how to fix the offense, please
  ask a teammate.

• When reading data, write tests for nil values, empty strings,
  and invalid formats.

• When calling redirect_to in a controller, use _url, not _path.

• When adding user data to the session, use the user_session helper
  instead of the session helper so the data does not persist beyond the user's
  session.

• When adding a new controller that requires the user to be fully
  authenticated, make sure to add before_action :confirm_two_factor_authenticated.

[stevegsa]

Don't we need to do this in two steps so we can rollback? If we roll this out in one step and have to rollback the code, the legacy code won't be able to decrypt the new sessions. A feature flag would control when we set new session encryption on (ie after this pr is deployed with the feature flag off).

[jmhooper]

Yeah, this is the first step. It still uses the legacy encryptor to write the new sessions, but can read both old and new sessions.

[jmhooper]

So you can go forwards or backwards with this one. The same thing goes for the next release which starts using the new method to encrypt sessions.

[stevegsa]

LGTM