First pass at deleting projects in the background.

CHANGELOG

app/controllers/projects_controller.rb

app/models/project.rb

app/services/delete_user_service.rb

app/services/destroy_group_service.rb

app/services/projects/destroy_service.rb

app/workers/project_destroy_worker.rb

db/migrate/20160122185421_add_pending_delete_to_project.rb

db/schema.rb

lib/api/projects.rb

spec/models/hooks/system_hook_spec.rb