Update base Event to send transit_agency property

[thekaveman]

Our core.analytics.Event class sets a provider_name event property AND user property to the (long) name of the user-selected Transit Agency. Since we only support a single Transit Agency right now, this property has seen limited use from the analysis/dashboarding side.

As we start to add some additional event properties e.g. in #342 - we should ensure all properties are named clearly. provider is used in other areas of our app (specifically the AuthProvider model) and could be confusing later on.

Unfortunately we have a bunch of events data in Amplitude already, and there's no way to update it. We could look at updating the ingestion pipeline in data-infra to transform that column name when we import into GCS/Warehouse/Metabase.

Acceptance Criteria

• [x] The provider_name event and user properties are renamed to transit_agency
• [ ] The Tracking Plan in Amplitude is updated to hide the provider_name event and user properties, and add the transit_agency event and user properties
• [ ] ~All data in Metabase shows transit_agency instead of provider_name.~

[thekaveman]

Merged the initial work in #901 to get the new property in there.

Still need to adjust the Tracking Plan, but we can probably not worry about backfilling/cleaning up our old data for now.