Refactor EnrollmentFlow model so in-person eligibility view can use copy from the selection label template

[angela-tran]

Original context is #2317 (comment).

Basically we want the radio buttons in the in-person eligibility form to use the same copy that the Benefits app uses in its enrollment flow selection form, but that copy is currently embedded within the EnrollmentFlows' selection_label_templates, e.g. "Older Adult", "US Veteran", etc.

Let's discuss how to refactor the model to achieve this, and then do it. It would be nice to not have to add another field to the model.

Acceptance Criteria

• In-person eligibility form uses the desired copy
• No regressions to the Benefits enrollment flow selection form