Model refactor: EnrollmentFlow Amplitude updates

[thekaveman]

After #2236 is complete, we need to get a new event/user property in Amplitude called enrollment_flows, to replace eligibility_types. This property is used fairly extensively in charts etc. These will all need to be updated, with historical data back-filled in Metabase with this update.

The EligibilityVerifier class no longer exists, so we need a new approach to the eligibility_verifier property. We can reuse logic from #2267 to generate this value.

Acceptance criteria

• Analytics events that include the eligibility_types (event/user) property replace this with (event/user) property enrollment_flows
• New property enrollment_flows defined in Amplitude
• Historical Amplitude events have eligibility_types and enrollment_flows properties coalesced into the new enrollment_flows in Metabase
• Analytics events get a value for eligibility_verifier (event/user) property in the same way as in Model and capture local enrollment events #2267
• Analytics event selected eligibility verifier is renamed selected enrollment flow
• New event selected enrollment flow is defined in Amplitude
• Historical Amplitude events are renamed to selected enrollment flow in Metabase