Remove SiteUsers concept

[drewbo]

Prior to the introduction of organizations, users were given access to a site via the SiteUser model. We need to remove this concept (all models, tables, endpoints, and related code) from the database, API, and application

Acceptance Criteria

• [ ] SiteUser model is removed in production database via migration
• [ ] Authorizer and model scope code (.forSiteUser) is updated to only reference organization membership as the site access condition
• [ ] SiteUser audit event is removed
• [ ] SiteUser Redux actions, reducers, and associated front end code is removed
• [ ] Rewrite loadBuildUserAccessToken to check organization users rather than site.Users for a valid access token
• [ ] Remove or rewrite all associated tests