Add Support to Session endpoint to support any valid picklist value

Presently, the API will allow only two values in Patch or Create: Writing and Soccer.
There are two other values in the picklist which need to be supported.

If an invalid value is submitted, the error response should include an easily enumerated list of all valid options. This will enable the application to self-configure to provide the user with only valid values.
Requesting Ticket:
https://github.com/AmericaSCORESBayArea/AmericaScores-CoachApp/issues/75