Participant roles should be enumerated values

[ttfkam]

Episode statuses should be an enumerated type within the database. This will prevent typos from creeping into the DB and allow for the UI to be populated with enum values from the DB rather than hard coded in the app layer. Also takes up less disk space and can be queried faster.

e.g.

CREATE TYPE gs_role AS ENUM ('new', 'waiting', 'pending', 'live');

ALTER TABLE participants
ALTER COLUMN role TYPE gs_role;