Medicare cardholder: create template for EnrollmentFlow.help_template

[angela-tran]

While working on #2356, I noticed we don't have a help template for the Medicare enrollment flow.

The copy shown for a flow's modal and help page section are always the same, but we do need to define a separate help template in addition to the modal template, e.g. modal--calfresh.html and help--calfresh.html.

modal--medicare.html was created in #2366; this ticket is to create help--medicare.html.