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, Site Users column is removed in production database via migration
• Authorizer and model scope code for builds (.forSiteUser) and for site (.forUser) are 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