Remove user activities table and use redis instead of PG for recording activities

app/models/user.rb

app/models/user_activity.rb

app/services/users/activity_service.rb

db/post_migrate/20161128170531_drop_user_activities_table.rb

db/schema.rb

lib/api/helpers/internal_helpers.rb

spec/controllers/sessions_controller_spec.rb

spec/factories/user_activities.rb

spec/requests/api/internal_spec.rb

spec/services/event_create_service_spec.rb

spec/services/users/activity_service_spec.rb

spec/support/user_activities_helpers.rb