* master: (26 commits)
  Remove extra subscribable_type filter on migration
  Add feature spec for labels subscription
  Avoid code duplication for label subscription status on label partial
  Rename LabelSubscription javascript to ProjectLabelSubscription
  Fix label subscription menu on small screens resolution
  Allow users to subscribe to a group label at group or project level
  Use button instead of an icon to subscribe/unsubscribe to labels
  Add CHANGELOG entry
  Add toggle_subscription action to Groups::LabelsController
  Allow subscriptions to be created without a project
  Use subqueries instead of joins to migrate subscriptions
  Add helper method to toggle label subscription on labels controller spec
  Remove default value for `project` argument on subscribable concern
  Use @project as default on ToggleSubscriptionAction concern
  Allow users to subscribe to group labels at project-level
  Pass project to Entities::Label to check if user is subscribed
  Fix specs to pass a project when creating subscriptions
  Refactoring label subscription toggle button text to accept a project
  Refactoring label subscription status to accept a project
  Refactoring notification service to find subscriptions per project
  ...