Only try to return a link to piv/cac if enabled

[jgsmith-usds]

Why:
The presenter uses the link form in the available alternative
options. When the piv/cac feature isn't enabled, this throws
an error.

How:
Guard the call so it doesn't try to construct a link if the
piv/cac feature isn't enabled.

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
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• For encryption changes, make sure it is compatible with data that was
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• For secrets changes, make sure to update the S3 secrets bucket with the
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• Do not disable Rubocop or Reek offenses unless you are absolutely sure
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• When reading data, write tests for nil values, empty strings,
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• When calling redirect_to in a controller, use _url, not _path.

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

[davemcorwin]

LGTM