[LG-257] Use redirection to hide nonce from HTML

[jgsmith-usds]

Why:
We don't want the nonce discoverable by JavaScript.

How:
We use an endpoint to create a redirect to the
piv/cac service. We create a new nonce when we
send the redirect.

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.

[monfresh]

Are there any security concerns with this method being executable via a GET request without needing to be authenticated?

[jgsmith-usds]

maybe -- it might be better if we required some kind of authentication -- either that we're at the point we want a second factor, or we're ready to set up a second factor. I'll add that. Have to do a rebase anyway.

[jgsmith-usds]

Rebasing brought in a before_action :authenticate_user! line. Manual testing seems to work -- without logging in, I'm put back on the login page, and I can add a piv/cac during account creation.

[monfresh]

LGTM